I'm not an expert with the 180, but we've had a Sterling in the family for years, so I have a little knowledge on 'em...
First of all, visit the Unofficial AR18/180 Website: [url]http://www.sterlingwebpages.com/ar180site/ar180home.htm[/url]
Regarding the price, that's probably around the going rate for a Sterling. It's actually lower than what I've seen lately, but I'd say you did okay. That is if the condition of the stock isn't too bad. I really don't know how to help you with the stock problem other than maybe to somehow add some material to the stock locking plunger, check its spring, or "tweak" the surface which has the dimple (ie: just bend the damn thing). I'd probably check on the condition of the spring first, though. If it's weak, that's something you could probably fix with a trip to the hardware store.
AR180 mags have the mag catch slot on the opposite side of the mag body, and the slot is more narrow and elongated than that of an AR15 mag. Other than that, they're nearly identical. You can actually modify AR15 mags to work in the AR180, and there are instuctions on that website I posted.
I personally wouldn't refinish it unless it needed it. If it does need it, I'd probably send it off to have it Bear-Coated ([url]http://www.bearcoat.com[/url]). There's a few people here on the board that have given them the thumbs-up.