take it a saab mechanic, have 'em look it over. should be kinda cheap, since they enjoy the prospect of your buying the car and bringing them more business. most can tell just by quick inspection if the car is ok. know several people who have/have had the saab 900 turbo, one with '88 specifically. no unusual troubles with the turbo feature.
fixing stuff on saabs is going to be expensive anyhow, but most true saabophile mechanics are quite resourceful and can think outside the box to get the best/cheapest fix. they just enjoy getting a saab back on the road.