First off, you usually can't go wrong with any of the SC's, V6, or V8, manual or automatic, they're all really reliable and good.
1) What are you thoughts on which one to get? (or which one to avoid, etc.) Avoid the one's that have major problems, with a mix of mileage over 230k. If they seem like they haven't been taken care of, that probably means the driver was a lead foot also. :)
2) How reliable are they going forward after a 100k+ miles (all the ones I can find are around there) They keep going. I see many MANY SC's, V6 or V8, surpass 200k like nothing. There was one that had 500+k and was still going.
3) What things should I look for when buying? Depends were you live, but what I'd look for is frame damage, look behind the seat belts and look for flooding signs, look for major rust, and check the bushings, and also make sure to drive it till it's warm. That's usually when the monster in them come out if they have problems. (like TPS, MAF, IACV, ect)
4) What things can I look forward to replacing in future? Tranny mounts, catalytic converters, o2 sensors, spark plugs and wires, a tune up, TPS, MAF, and the IACV. Not all, but these are popular problems on older SC's.
5) How much is the maintenance per year? IDK actually. I've had my SC for a little over a year now and it has cost me 1600 so far to restore and it's near immaculate condition now.
6) Also, besides Autotrader/Craigslist, where is good place to look for these cars? Your state magazine, though, you have to be johnny-on-the-spot immediately. eBay is a good place, search local, or even go out of state to grab the right one. Many SC'ers do that and I will be in the mere future.
Hope you find a nice SC! Enjoy it!