My next project will be an FD or RX-8(and I passed on a few FDs getting my FC), so I kinda know where you're at.
Make sure the car has no frame damage or was repaired by a reputable frame shop and the owner
has receipts. If he doesn't get the frame checked before you buy, he shouldn't have a problem with it. If the frame shop doesn't have a problem then you shouldn't either.
Manual swaps are pretty easy, and if you can't find the parts in a junkyard they're aren't all that expensive. Worse comes to worse you can order the clutch master and slave, lines, pedals and driveshaft somewhere and get a Jspec which will come with the tranny. Then you have an extra motor to send out for serious porting and rebuild when you want some good power.
Don't listen to people who havn't owned an RX-7, because they don't know WTF they are talking about. Take care of the FD and she'll let you beat on her forever. Replace the old hoses with silicon and try to get rid of most of the rat's nest, do the reliability mods first, that's what fails on these cars, not the engine itself, but the overly complex peripherel systems, so simplify and you'll be in good shape. A motor in good shape running stock boost can run you over 100K miles of serious use. Look at fuel computers and pumps, you'll want to tune it, trust me you won't be able to help yourself, haha. So get the fuel stuff in place.
as for the price, well that varies greatly depending on the owner, where you are, etc etc etc. but I think that offer is pretty fair. I've seen really nice FDs with good tuning go for ~9k, and I've seen shitty ones go for 18k, just gotta look and see how things are in the market. The KBB and NADA values don't matter shit, just like with 240s. My FC probably isn't worth shit in there but I talked to some kid who bought an identical car for $5k and thought it was a steal, so...
also, lurk and post on RX-7 forums, I don't know how many other 7 owners are on a forum like this, so it probably won't do you much good. They'll tell you silly things like, "OMFG so unrelaible it will BLOW UP!" or "GET ANOTHER CAR!" or other stupid shit.
also, I'm not the hugest fan of "the evil forum" but here:
3rd gen FAQ + buyers guides
I hope your negotiating goes well, you'll LOVE the 7.