Used gun prices vary a good bit according to where you are. Around here, a 2.75" chambered 870 as you describe would be priced roughly $125- 150.
Call Remington at their toll-free number, work through the phone tree and they will give you the year your new family member was manufactured. It's an interesting bit of trivia to know about your gun.
As to parts, don't worry too much about that. There likely isn't anything that needs replacing on your gun, no matter how it looks on the outside. Look at the owners manual on Remington's website for field stripping instructions, and break your gun down into its component parts for a good cleaning. I have found enough grass stems and weed seeds in the receivers of old 870s to make a birds nest out of in the past- heed what the old phart here is saying, OK?
You need to get really familiar with your 870. Its modularity makes it easy to field strip and clean, it needs to not be an issue with you to field strip it. DO NOT however go any further than field stripping- once you get it down to the major components as described in the owners manual, there are no user serviceable parts inside. Not till you learn more, anyway 8^).
Resist the impulse to buy one of every 870 accessory out there. Shoot it as it is for the time being. Shoot it A LOT. Oh, certainly you should make sure it fits you properly- any decent 'smith can shorten a stock and install a good recoil pad as needed. And if you anticipate family security duty for your new/old friend, perhaps a shorter barrel might be in order. Those are pretty easy to find, no need to go whacking off what might prove to be a really good wingshooting/claybird barrel when there are lots of factory built 18- 20" tubes available. Shoot it as it is, find out how it patterns and handles, then make decisions on what you want to do with it.
Please don't be one of those poor lost souls, the MySpace commando, who spends $$$ and time getting an 870 'just so' and picture perfect- and NEVER shoots it. Go shoot the thing, OK?
And be safe out there, while you learn what generations of shooters before you have come to know about 870s.
lpl/nc