Water pump - helped a friend with his - not that hard - took an afternoon.
Battery - about $100 at Walmart - 5 minutes to swap
Valve cover gaskets - never did it but I don't think it's a big deal
Belts/tensioner - nothing special - no tougher than any other car
Valley pan - I could be wrong, but I think these are more of an issue on the newer valvetronic motors.
Alternator is quite pricey (I think $200+), but my 540 alternator was still original at almost 300k km
Powersteering pump - I don't think I've ever heard of this being a common failure.
As for a car with no history, it is what it is - get an inspection. Check the fluids. A well maintained car would at least have fluids in good condition - esp the oil. My experience with these cars has been awesome. Esp with advice from forums like this one, I've saved tons of dough on repairs. I find that these cars are easier to work on too. Parts aren't seized up as much, not as many scraped knuckles.... I was helping my dad with his Caddy today and I was rudely reminded of that!