Help in purchasing a MBP

Not sure on how much they are worth

Yes it will run Mountain Lion

Personally I'd go for something more recent, depends on what you want to run on it though

I wouldn't give more than $300-400 depending on if it is pristine condition.
A Mac with a Core 2 Duo should run Mountain Lion, no problem.

Personally I would buy 8GB RAM and at least a 120GB SSD to drop in it and it should be rock solid for a few more years.

If you need pure processing power or will be using it for anything other than mainly internet surfing, emailing, word processing then I'd probably look for something with an i5 + CPU but it's all in how you will be using it.
My wife was still using my '09 MBP up until she went and bought a rMBP cause she wanted a lighter laptop, 15" screen and her retina ipad ruined her for any other type of screen.

Assuming you are bidding on ebay? If you look on the right side of the search results, you can filter by 'completed listings'. This will show you what people were able to buy them at and what they are bidding for them but not winning. Should give you a pretty good idea of what they are worth