not sure about the motor prices, but on the mr2 you have to be careful with the motor and the transmission, if you get a model with higher mileage, and swap the motor out the tranny might not hold up as nice as you would like

so still getting one in your price range but with lower mileage is going to be best, and i would stick with a manual shift gate if possible! stay away from automatics if you plan on supping it up