600-700 USD is a decent price for a 4th generation Summicron, IIRC. They can be found on Ebay and you can sometimes get them cheaper than that depending on when/where you're looking. User lenses, those that aren't in great cosmetic shape but are fine optically, tend to go for less than mint.
The Summicron has a reputation as pretty much the greatest 50mm lens. I haven't gotten to shoot with one yet, but my time will come. I suggest it. If you can't do the Summicron, the Nokton is a great lens as well. It's faster, too, so you'll be able to shoot handheld in more situations. The drawback is that you have to get an adapter to use it on an M-mount.
The choices are there. Both the lenses have good reputations. What kind of shooting do you want to do with it?