easiest way to get the power your wanting is by tuning the 3S-GTE, and it will be the cheapest by far. supposedly the stock bottom end is one of the weak links to over come, but shouldnt be a problem with 300hp at the fly.
as for cooling, there are a few options. the one i would be looking at personally, is a liquid to air exchanger. if you've seen the video of the GTTT, you'll know they can work well, try to emulate that system, although price will be a bit of an issue here, overall it will still be far cheaper IMO. even still, there should be other water-air set up's for MR2's.
my advice now is to look to jap tuning. i've seen a few MR2's in the tens, on street tires, look there for advice.