I'd say struts. Struts go bad on these cars pulling out of the lot!
3.1'* had intake plenum gasket issues, rod knock issues, and are notorius for clogging injectors if you run them on cheap fuel (not 87 octane mind you, but fuel from some no-named gas place).. 3.8'*, well, you have a 3.8. The L36 is an L36 regardless of what platform they're in. The 4T65E has a history of weakness, even on non-boosted apps, so be weary on that one.
But really, nothing more than standard automotive things to watch out for. If you were looking at an earlier yeared 3.4L "X" motor, I'd tell you to start running!
The cars themselves are pretty stout and safe. I've had Lumina'* (a 95), Monte Carlo'*, Grand Prix'* and they've held up really well in accidents.