Dude, sweet 870, and crazy timing! Were you just looking for a shotgun, or are you planning on hunting or something? I'm jealous of the vented rib on the barrel! I wanted to get one, but couldn't get that and the stock I wanted together, so I'll have to replace my barrel eventually.
If you want to replace the wood furniture with black polymer, there are several vendors. Knoxx Tactical is very well regarded for their collapsible stocks. In addition to the obvious benefits of a telescoping stock, the Knoxx models also reduce the recoil considerably. That could be very beneficial if you plan on putting a lot of rounds down range.
As for the Carbon 15, they're very decent Bushmaster ARs that really run the gamut in terms of configuration and options. Carbon 15s are available in pistol and rifle configurations in a variety of pistol and rifle calibers. You can see the current lineup here:
http://www.bushmaster.com/catalog_carbon15_index.aspOnce you've found your model, take a look at GunBroker.com. That site will give you a decent idea about the reasonability of the asking price for a used gun. As long as the gun is in decent operating condition and it has the features you want (e.g. caliber, barrel length, receiver type), I'd say the price you quoted sounds incredibly reasonable.
-b0b
(...yay AR15!)