Nik_Lee
I do like the sound of the 2AR-FE that Knightrous suggested and I also like the idea of being able to run a EA60 6 speed with it! Out of curiosity which box would be stronger the EA60 or the 5 speed that is already behind my turbo?

Paul Woods from TwoBrutal has stripped down both the EA/EB and E series boxes and rates them as having a similar capability as the E153.

While looking at alloy camry engines like the 2ar, you could also consider the 2azfe. Plenty common due to them being used in 2 series of camrys, as well as rav 4's, there are plenty of parts for them available, you can buy stroker kits, sleeves, even 1000hp crate bottom ends for them. Not as advanced as the 2ar, but a lot cheaper.

Nik, is your car a genuine Gen3 Hardtop turbo import? If it is, do you really wanna do an engine conversion on it? They are quite rare as it is.
 
I wouldn't say the Gen3 3sgte block is weak, its just not as strong as a gen2 3sgte or 5sfe etc. You can still get big power from them. How much HP are you aiming for in this car?

Yes my car is a genuine Gen3 hardtop turbo import and I did look for quite a while to find one that ticked all those boxes, preferably I would like to be able to just stick with the gen3 3S but I don't want to go building a strong engine and have the block fail once it is under a bit more stress.
 
To be honest I would love to eventually get the car somewhere around 300rwkw.

If your goal is 300rwkw then all you need to do is build up a 5sfe block (rods + pistons) and swap over the gen3 head and oiling system. Then you end up with a strong block with capacity increased to 2.2L
No need to do an engine conversion for 300kw. Unless you want the low end torque of a V6 with 300kw :) 

Ok so far most people seem to agree that for a reliable 300kw car your easiest option is to swap to built 5S bottom end, so my thinking is if you have to swap to a different short motor and you do still have to modify it for the intended purpose then why not explore other options. If you did go with something like the 2AR like Knightrous suggested or possibly the 2AZ and build that up could you possibly be gaining a reliable 300kw with the added bonus of a lighter weight engine and the option to run a 6 speed box.
Just thinking it may be food for thought if I had to look at buying a 5S to achieve what I wanted anyway.
 
And there seems to be a few people starting to play with the 2AR so I'm guessing there will be more and more performance parts becoming available for them.

If you wanted an in depth look at a 2AR engine, have a read through this article:
 
http://www.motoiq.com/MagazineArticles/ID/3102/Nerds-Eye-View-Inside-Fredric-AasboPapadakis-Racings-Amazing-V8-eating-2AR-Toyota-Engine.aspx
 
It is a look at the build up of an 800whp 2AR that Fredric Asabo uses in formula drift in the US. there are actually another two cars that run 2azfe engines as well.

Thanks for the link!
I had seen some slight coverage on that car in Speedhunters recently, it sounds like quite a good engine.
 
http://www.speedhunters.com/2013/04/papadakis-racing-tc/
 

I hope you have a decent budget for this project as you will need it! It will start at 5k for the V6 build then head upwards at a aggressive rate when considering turbo charging, built bottom ends etc.
For around the same money as a serious AR turbo or 5sgte (10k+) you could build a 2GR un-boosted with a figure close to what you are seeking and have a bomb load of usable power where it counts. Infact, on E85 I dare say you could get there. I personally love the turbo style of driving but after driving a 2GR race car recently (Thanks Pete!!) it really makes a difference and the sound is also very hard to beat. 
If you add some turbo or SC (both around the same money but less heat issues with the SC) you can achieve 700hp on E85 with a conservative set up. 1300hp has been achieved with 1.4 bar at 7000rpm. Budget for this kind of weapon is 25-30k being professionally built by some of the best in the country.  
I have no knowledge of your engine( will have to do some research at some stage)but the first thing to do is 'have the head flowed' to see what it can produce and what it required to get the flow rate balanced for your application. At least then you will know what your current engine is capable of for a few $100 before going crazy with something else. Also need to know how strong the bottom end is. The 2GR bottom end is standard in all these hi-po engine builds. Rods and pistons are replaced and the heads receive decent prep along with a number of other undisclosed tweaks to confirm reliability at sustained high rpm runs. If your bottom end is the same (6 bolt mains, forged crank, forged rods , etc) you may as well stick with the 2.7ltr as it will save you a lot of work/$$$.