Anything to watch for when buying a 2009 MBP?
I used to work as an Apple Hardware Technician, and the biggest non-damage related issue I've seen with those machines is usually the battery (which is not technically user-replaceable). I'd make sure that it runs for at least 3-4 hours on a full charge. Also make sure that the optical drive works, because I've seen a number of those go out, as well, but that can be said for any notebook computer.

You will also want to check for the normal stuff: Impact to the casing, cracked screen, liquid damage, etc.

That seems like a pretty good deal, however. It looks like the average price on eBay is around $600, and that machine will have plenty of power for your purposes. Heck, I still do development on a 2009 Macbook and it works great.

For future reference, you can always find a listing of the open repair extensions (which are basically known-issues) at Apple's website.