Buying any used turbo 'sport performance' car (WRX, STi, MS3, MS6, Evo, etc...) second hand is a really BAD idea unless you knew the previous owner, have a lot of cash, or have a lot of time to wrench on your own car.
If they grant you a two day no-obligation test drive, make sure it's a NO-OBLIGATION test drive; not one of those, you break it you buy it deals. After about 70 miles of normal driving, drive the hell out of the car, redline, rev-match, hard corners, figure 8's, etc... Then do a leakdown/compression test, find a local mechanic shop with a lift and look for any leaks, odd sounds, etc... If there is no CEL, or odd noises, then the car is 'okay' to buy if your heart is set on it.
Like others have said, we haven't seen the car so we're just basing our advice off what you tell us (granted there are the Speed6 owners that know better than me). In the end, it's all up to you.
I'm not talking out of my ass, I've done work on a MS6. We replaced the turbo, repaired the driveshaft, and the engine just got rebuilt because seals went bad; and I think piston rings or something crapped out (I wasn't there for the engine rebuild); then there's the regular maintenance of wear and tear items.
Said car. It has less than 70k miles and well kept. I'd buy this one no questions asked because I know the full history.