I would look at a 2000-2005 Buick Park Ave. I have a 2004 and love it. Tons of room, comfortable ride, and the GM 3.8L engine will run forever. Seriously, 250-300k miles is not uncommon for these engines. Plus about 26-30 mpg on the highway. You can find them for under 4k all day long.
Anything with the GM 3.8L engine. They can be found in:
every single GM V6 will need the intake gasket replaced. if the PO let it go too long, say hello to a ticking time-bomb.
If you do go for an impala, monte carlo, or any other GM based car with the 3.1 or 3.4 v6 engine, make sure it has had the intake manifold gasket repaired. This gasket fails on every one of those engines and its a pain to replace, costly if you have a shop do it. Just food for thought.