what would be the best choice to upgrade an 1991 toyota mr2 2.0 turbo?
aftermarket turbo? any suggestions?

http://www.mrcontrols.com/primers/power.htm for the basic power stuff

I'm a fan of upgrading in this order:

suspension, wheels/tires, intake and exhaust, intercooler, and then if you want to spend a lot of money, turbo/ecu.

But, it all depends on what you're looking for. lots of power, quick power, cheap power... etc.