I purchased my Mustang with "unknown" miles. The odo read 107K, I later found out through Carfax that it was reported "unknown" on the title like 5 years before I bought it. So I'm sure it had A LOT of miles before I got to it.
I had already planned on replacing a lot of things like the suspension and brakes before I bought it (I was going to use it as a road race/auto-x car), so I wasn't as concerned to the condition - as long as I could drive it home and it was straight. I've worked on it quite a bit since buying it (I bought it for a little over $2K). I would say plan to replace certain things, like the rotor/cap, pads and brake fluid, oil change, maybe motor mounts, fuel filter, air filter, plugs and wires, stuff like that. Depending on how it has been taken care of (or recent mods), the suspension might be in good shape. Mine was okay, the big problem areas were all the bushings, most were rotted out. The car drove okay, it made a lot of noises (groaning and such because of the wiggle room from the bushings). Make sure the car is straight, check all the seams in the body panels - different sized gaps imply body work/damage. I got lucky with the engine; while the POs neglected the interior and paint, they apparently took excellent care of the engine with regular oil changes. My car also had different tires on all 4 wheels, none were the same, implying the kid I bought it from didn't have a lot of money to take care of it, so what else did he gloss over... Also plan on a LSD replacement - you may not need it depending on whether it's been done or how it's been treated. On my car, the Trac-Loc was shot. It was pretty much an open diff. The water pump gasket was shot, and the timing chain also needed replacing.
In summary, it was in fairly rough shape - dirty and stuff like that, but overall not too bad. At that mileage, there are some "big" things to do, like timing belt/chain, maybe the clutch is getting worn, how's the transmission shifting - worn synchros? There are also a lot of minor things to do, like pads and fluids, but you should be planning on doing those anyway. Ultimately, how well do you think this car was taken care of? Is it clean, good looking paint, something that might imply that the PO took pride in the car and thus probably took care of it? Or is it dirty, crappy paint, sloppy fixes (like only 2 bolts holding the driver seat down) that might imply that the owner could care less. If it's the latter, then you obviously run a bigger risk of something being an issue. I took a chance (my Mustang was the latter, it looked and sounded like shit) but got lucky. The engine is stock and strong. This was okay for me since I was replacing just about everything anyway, so this was the perfect car for me. If you are looking for something you can buy and drive with little extra work, then it's obviously more important the car looks well cared for, things replaced, no broken items, etc.
