Interior pieces are not too hard to come by, but you'll find that they nickel & dime you into quite a bit of cost after awhile.
Your pics didn't work, but if the car is in "good shape," with all the work done to it that you listed, it sounds like a very good deal at that price.
the most common spots for rust will be the front quarters right under the "MUSTANG" letters; the doors anywhere along the bottom edge, but in particular the front corner; the rear quarters all around the fender area and in between the wheel and the bumper; look carefully inside the trunk area for evidence that body panels have been "massaged" to repair accident damage; look inside the trunk lid along the edges, in particular where the "MUSTANG" letters are on the deck lip; also look at the front window pillars where they meet the roof & at the rear pillars where they meet the fender; check all along the edges of the front and rear window glass; see if you can lift up the carpeting in the front footwells to check for rusted-out floor pans; pour several cups-full of water through the cowl vent to see if you get leakage into the driver's and passenger footwells; look up inside the passenger side front fenderwell to see if the batter tray is rusted through; check all the frame rails for damage & rust.
I think those are the major problem areas on classic Rustangs, and you said you know how to check for rust, so you should be fine.