aite...u got some decent questions and i got some legit answers.
1) turbo is the way to go. N/A is mainly for AutoX, but i'm assuming you need a speed demon for the streets. N/A lacks the power and torque (equaling fun..fun...FUN) that the turbo offers.
2)MR2's are expensive...obviously. 91-93 turbo are usually in the $6k-8k range depending on condition and mods. most of them will be over the 100k mark on mileage, but that's not an issue with Toyota's if they have been well-maintained. Expect to pay up to $18,000 for a nice 94 or 95. but as far as I know, '91 are the lightest and most fun at the tracks in terms of behavior at the limits. N/A are usually $2k-3k less in cost compared to their turbo counterparts.
3)Modifications. it's up to you. if you like to spend lots of money and fix everything that goes wrong with the car, then buy one that is mostly stock. if you just want to dominate the streets NOW, then there are lots of MR2's that are already modified for good deal...just look around. xcars.com, and mr2oc.com are great sites.
4)as far as comparing an MR2 to an Integra, i believe that was on the board before, and mostly everyone agreed that the Integra is a grocery getter compared to the MR2 (turbo).
now that's jus my thirty-two cents.