kvnMac,
I bought my 13-inch Core 2 Duo Mid 2010 MacBook Pro used from a private seller. The one difficulty with my purchase was that the seller included the wrong set of grey installation DVDs with it, and he didn’t have the proper set for it among his collection of grey installation DVDs, so I had to purchase a replacement set of grey DVDs for it elsewhere. If you’re not comfortable with troubleshooting a computer in case of problems, then you might prefer purchasing a refurbished model from Apple, an authorized reseller, or a reliable local refurbisher; it’s not likely that a private seller will offer support to you in case of problems beyond “dead on arrival”.
For personal documentation, browsing, and e-mails, an older MacBook Pro would be fine. For movie watching, do you mean watching online, watching on DVDs, or both? My guess is that an older model would be fine for both kinds of watching as well, but I’m not much of a movie watcher myself, so I’ll let others comment on that. For these purposes, the amount of RAM is probably more important than which CPU it has. My MacBook Pro has a 1280 × 800 display, and I find it enough for my needs; if you have a nearby used computer store in which you can see the 1440 × 900 and 1680 × 1050 displays side by side, that might help you to decide which one you would prefer. The 15-inch displays also come in glossy and matte finishes; some people have strong preferences one way or the other. My 13-inch display only comes in a glossy finish, and for me, it’s fine.
If you prefer a 15-inch display, then you should avoid the 15-inch Mid 2010 model — this particular model is known to have a defect with its NVIDIA graphics card, and Apple no longer services this defect gratis.
My preference was for a model that came with grey installation DVDs; the more recent models (Late 2011 and newer) come from the factory with their reïnstallation software on a hidden part of the internal hard disk, and in case of trouble with that part of the hard disk, or upon replacement with a new hard disk, one can reïnstall the OS over the Internet from Apple servers. If you live in an area with excellent Internet service, then the Internet recovery feature of the newer models could be preferable to you. I don’t, so I preferred having the DVDs, so that I could reïnstall as needed without depending upon Internet access.
As far as a preferred OS version, mine originally came with Snow Leopard, and currently has Mavericks on it, and both have been good in my experience. (Some people on the Mavericks forum here have reported problems with the Mail application, but I don’t use it, so those problems haven’t affected my usage.) I haven’t tried either Lion or Mountain Lion, so I can’t comment on those versions. If you have a particular software package that you would like to use, that might help to form a more definitive idea of which OS would be best for you.