Hey mate,
I can give you some advice with the turbo side of things, but please note my experience with all of these turbos are pn motors other than 3sgtes. In general i find the 3s to be a touch harder to reach power numbers than nissan and othwr equivelants.

Gtx28 series:
-awesome turbos, good response, and good power. I strongly recommend the .84 rear housing. The gtx2863r is comfortable in the 200-230rwkw range, anything more than that you need to work through flow restictions etc when trying to reach above this level, and you will be pushing the turbo north of 20psi.
- the gtx2867 is also a great turbo, for being a touch bigger, the lag difference is generally 2-300rpm more than a gtx2863r. Again the .84 housing is recommended. Comfortable working range is 220-250rwkw. These turbos have seen in a few circumstances as high as 310kw, but this is on 25+psi boost.
Gtx3071r
- a great mid frame turbo. Maybe a little aggressive with a .63 rear housing for an mr2, so again i will recommend the .84 rear housing. This turbo is in the comfortable 250-300kw range. Expect another 3-500rpm of lag on a gtx2867r. Has a 4 inch compressor inlet, which may cause some extra plumbing work if you run an airflow meter.

As for the internal vs external wastegate:
- higher initial expense using external wastegate
- more joints for potential leaks with external wastegate.
- external wastegate w/ screamer pipe is a fire hazard if care it not taken with plumbing.
But:
- external wastegates are easier to dial in a setup with changing springs, and depending on brand are completely servicable.
- running a screamer pipe setup has the same effect as running a larger diameter exhaust, as not all exhaust has to flow through the exhaust.

The working figures i am using for the gtx turbos in the post are largely with my experience using them on nissan sr20 engines. By comfortable power ratings i am referring to basic bolt ons, not always with uprated camshafts and operating in the 15-20psi boost range.

Hope that helps.

Check out the GTX 3067, yes 67 not 76. Could spool faster and outflow the popular GT3071R upgrade that is tried and tested on the MR2. Check out this thread on the OC, his GT3071 died and he replaced it with the GTX3067 and gives a great comparison.
 
http://www.mr2oc.com/showthread.php?t=472099
 
 

Stay away from the Garret GTX as they have poor bearings. The bearing will fail when pushed. It then drops the broken bearing down the oil return line then ends up destroying an engine. After this happened to my engine, I did some research to find it is a common problem. The cores are made to be replaced not rebuilt so there is another expense you can do without. I spoke with the Garret rep at WTAC about the issue. He stated you need 'race spec' bearings if you wish to push the turbo or the plastic liner will fail then the bearing fails.
The 30 series is too big for trackwork based on your spec of car and the tracks we use. I ran the GTX3071r/.02 housing which was woeful on short circuits as you need to use 2nd instead of using 3rd then punching out of a corner with the useable torque you will find in a smaller turbo. The cost of fitting the external gated turbo with exhaust manifold was also prohibitive. The heat from this turbo was also ridiculous. There are big heat management issues with the bigger turbo too. It can be done but its not cheap and its time consuming. Even at AC the other week your little standard turbo was glowing like a beacon. You can double it for the GTX after a solid run.
Something around the 28 series spec with internal gate running at 18lb(max)/ bush bearings or race spec ball bearing is what I would recommend as you do mainly 'track' the car. Will not kill the engine, will not heat up like a blow torch. It will more linier powerband at lower rpm. It also would be better suited to cams/manifold which you run in the car.
 

B24, i have only personally seen 1 gtx core have the plastic bearing cage fail, but i understand what you mean. On the other hand, i believe (i may be wrong) they have since upgraded the ball bearing system in these turbos to a ceramic bearing core.

I left the bw efr turbos out of my post, as i think they would require a manifold change to suit them being fitted.

There is a company i can refer you to if you want a proper motorsport spec garrett turbo, with upgraded cores, 360 degree clamps, inconel turbine wheels and upgraded heatshields, but their turbos are far from cheap, and they are based out of the uk.

I have to agree with kameleon on this one. A bolt on is the way to go. Whatever your budget it is, you might as well double/triple it (Im serious!!) if you are going for anything that cant bolt right up. The K1-380V is a great turbo and with a turbo beanie, ceramic coated dump pipe and and some basic mods to promote airflow through and out of the engine bay you will have a simple and reliable set up.
 
Keep the boost modest until you pull a few seconds off your lap time and are able to maintain consistent lap times and then you can feed in more boost for more power.
 
Keep it simple and most of all have fun with it!

I think Alex should contact Trav about the K series. Cheap and easy, bolt on, 230kw is plenty for a MR2.
With that much power, you should be doing 1.09 at Wakefield ,1.50 at SMP and around a 1.39 at Winton as that's what this car is being used for, track work.
Anything else will cost a bomb. Enough for anything between 10-20 trackdays and you would only pick up another 30kw so why bother. Amisavoy has finished his conversion to a aftermarket turbo and it destroyed him. Was off the road for 10 months, cost a fortune and was 2 seconds quicker than his 100kw NA. He is still having problems and may sell up so I really do not believe its worth all the effort of big turbo and all the grief unless it is a drag car or street bruiser but that's just my two bob and I would not like to see you suffer as I and Ian have done over the past few years.