Thanks chaps, I see some focus on insurance which I'm not a fan of, so there won't be any voiding of any policies.  
 I think I've made my mind up to buy a turbo version not needing too much work. Then if I can stop myself from driving it all the time, and remember that the whole project is an educational for my son who wants to be a car designer (used to be a monorail driver) , then we can put it up on blocks, work on the rolling chassis, suspension, centre of mass, etc. Then buy a spare NA engine, strip it, rebuild it and drop it in, ready for his L's. Turbo engine can sit pretty until he is legal, then we can put it back in, and fight over who gets to drive it. I am assuming that the internals of the turbo'd version (eg pistons, cams, ??) are different to an NA version, so just "unbolting" the turbo would not give a great result, but maybe that is wrong ??
Or my son may be happy to drive the Camry stationwagon until he is legal, ok by me, I'll drive the MR2.
He might feel jipped after working on the car, and then not be allowed to drive it.  :o)
 One more thing I'd appreciate your collective experience on is weight reduction, looking at bang for the buck, what replacements give the best gains ? What sort of saving can be had for $1k, $2k?
 
cheers,
Twodogs
 

I bought a NA SW20 when I first got my L's. The plan is to keep it NA while on my p's and focus on interior and handling before doing the turbo conversion later. ThaT wouwould be my suggestion

Good plan +1 to no conversion/headache route (personal experience hehe). Might spend more initially but you'll prob end up spending more overall if you cheap out at the start buying a lemon.
You will have to mess around with the compression ratio (to make it 'drive-able') if you decide to "de-turbo" the engine so I wouldn't do that...
No real significant weight savings on the SW20 can be done cheaply unfortunately. A/C, spare wheel, exhaust looking at 50-60kg tops I'd say. With 2k can definitely shave off a few more kgs.
 
Link to comprehensive weight reduction:
http://www.jekylhyderacing.com/weight.htm

Reading back through the responses, which I greatly appreciate, it is clear that unbolting the turbo is not an option, so if son needs an NA version to use the car then install a complete NA engine. 
Thanks for the link Tree. I will check it out. I also saw a thread on here where all the cars that attended a track day at Wakefield were weighed and presuming all had full standard fuel tanks and spare wheel removed, then there are interesting differences. 
 Ok now that I've decided to purchase a Turbo version, it seems there are plenty of aftermarket parts, and many questions so here goes....
 Does it matter if the car is not an imported factory turbo ?  I presume Toyota uses a fixed and conservative boost, not sure what compression ratio is used. Or are there specialist converters here in Oz that do a better job on the performance/reliability scale ?
 If non-factory is preferable, then are there converters and brands of turbos, computers, pumps, exhaust that I should be wary of?  
If a guy says " I converted it myself" is that a good thing ? 
I presume all privateers would get the final mapping done by some place with a Dyno, so there is another question...who are the best people in Sydney who can do that for me ? or do I trust that it has been done already so "just leave it alone" ? 
 
 
 
 
 

Don't be put off by the "did it myself" tag - but do the research, have a good look at the standard of workmanship on the car.  I've recently had the pleasure(?) of sorting out a few (??) issues on a DIY Turbo AW, and the level of bodge exhibited by that car - affectionately know as CurseDub - was truly spectacularly.  If you choose to head down this path, be super picky, spend a morning poking and prodding over the car, perhaps enlist the aid of a local forum member to find the problem areas.  We're all here to help . . .

Checked out the Jekyll and Hyde site, very good.
 
and thanks Dennis for the comment.
What is  the feeling towards standard imported factory turbos ?
Reliable / not necessarily,  enough urge / gutless, (ie not worth having the turbo) ?
Do they comply with ADR ? Do they have all creature comforts eg A/C ?
 
or is the verdict that you can do waaaay better with non-factory, just gotta wait and keep looking ?
 
cheers everyone, you have all been really helpful, thanks

That list of weights from Wakefield can be a bit inconsistent because of different trim levels and optional extras at the time of purchase. My car was weighed with the A/C stripped out, can't remember if I filled up before or after (I think it was at 3/5 tank of fuel when weighed). My car is a JDM G-Limited so it has a few differences from the ADM most notably P/S steering that obviously adds a few more kgs. Iain's car is basically a stripped out ADM (but still road worthy?)
 
I'd say factory turbos>non factory unless the non factory was owned by a good mechanic or someone who knows his stuff so I can't imagine seeing many of those on the market? You can always do cheap mods to factory:
http://www.mrcontrols.com/primers/power.htm
I can't speak from experience here but given the choice between Gen 3 and Gen 2 3SGTE I think most would choose Gen 3 as it's had quite a few upgrades from Gen 2, probably more reliable too (largely subjective to owner tho).
 
Whether it's got A/C or not just depends on the car mate :)

Have you considered the V6 path?
This could work quite well for you.
If you are looking for weight reduction the best place to start is with a hardtop fitted with a 3.0 V6. As for how far you wish to go with weight reduction is up to you. There are a few threads on the forum regarding both the V6 and the weight reduction options.
There is also a list of cars from the MR2 trackday where they were weighed-in at the start of the event. This will help in giving you a ballpark on what you are starting with and where your car could end up(weight).
If it was me, I would not bother with light weight panels for reduction. Its the most expensive way with not much gain. Lightweight brakes and rotor hats, light weight wheels and tyres are a good starting point.
Depending on how comfortable you would like to enjoy your daily ride will effect ow much weight you wish to loose. Removing A/C, power steering, replacing brackets with light weight units, trim all the bolts back, replace non load bearing bolts with alloy units, replace fans with light weight units, reducing the wiring system are all things you can do yourself and manage some good gains but can effect your daily driver comfort.
Good luck!

Hi 24,
 don't know anything about 3.0 V6 engines in an MR2. 
I know the engine I have one in my Camry Swagon. Do they drop right in ? Will the MR2 gearbox and drive handle the extra grunt, because I would definitely tweak the V6, (they are mildly tuned standard).
I believe the turbo models have stronger drive train and box.??

 
The V6 is a great option and it does fit without too many issues. Three are kits available and a few threads on the conversion with people ready to help if needed.
The 3.0 can be managed quite well by the NA gearbox and it can be adapted to the Turbo gearbox if required. There is a guy who races in the U.S. who fitted the 3.3ltr with VVTi heads with excellent results. Also sounds a treat. It is the same block with different stroke. This package will save quite a few kg's as opposed to the 3.5 V6 or turbo.
There are a few 3.5ltr(Evora motor)V6's SW20's in Aust and around the world with kits available too but it requires a lot more investment, similar weight to the 3SGTE.
One of the guys in Sydney has an excellent example with a 3.0 SC SW20. The best V6 conversion I have seen and very competitive at trackdays. Highlander/Peter, has a ex Ric Shaw built SW20 production sports race car with a 3.5ltr fitted. This is as quick as the 3SGTE's at the tight circuits. Not sure how it would fair at the high speed power tracks.
The last option (I can think of) is the 2AZ/2.4 ltr NA. It's an all alloy block/head/VVTi/light weight. It would be a great NA package and a proven performer with a hair dryer attached.
There is another option which the purist's do not favour. This is the Honda conversion (2.4 integra). Lighter, faster, exhaust on the correct side of the engine and a ton of after market support. You can also find info on this conversion.