Prices vary a lot depending on the area. Pacific Northwest and Colorado are more expensive, from what I have seen. Not sure where you are from, maybe Eastern Oregon or Washington (guessing). Best thing to do is look at what comparable cars sold for in your area.
It is hard to generalize about lifetime for these turbo engines, but 250,000 miles seems do-able with a turbo rebuild somewhere along the way. People that get long engine life tend to have conservative tune, do a lot of highway miles and maintain the car very well. What mostly kills these engines is detonation from a bad tune, low oil or lack of maintenance.
For power mods, consensus is that the "sweet spot" is a good quality downpipe with high-flow cat and a conservative ECU tune and stay happy. With stock turbo, you won't see much benefit from changes to the catback exhaust.
It is important to understand you should not do any power mods on these cars without getting the ECU tuned to match. That would include intake, exhaust, intercooler, injectors, fuel pump, wastegate changes, boost controllers, etc.
There's tons of info in the archives here. This just touches the surface.