I think I looked at about 7 Mustangs before I picked up the one I'm working on right now.
What I learned...
Look for rust. I found a father/son restoration job that was just horrible, but from 10 feet away looked good. The panels didn't line up. The rust was actually around the windows but was painted over. It was a matter of time before it just gave up. My car was almost completely rust free except for my rear frame rails. I didn't even *look* there but now I need to get it done before my leaf springs go flying off. I looked in the normal spots (floor pans, etc).
You don't have to get a V8. I just wanted a nice Mustang, I could care less about how fast it went (I have a 2010 Mustang if I need a bit more speed). But, I was so hell bent on getting a V8 for a good price that I probably overlooked a few nice I6s. I finally found my '66 on Craigslist after watching it for a few months. It started at 7500... went to 6500.. and finally down to 5000. No one wanted an Arcadian Blue I6... everyone wanted a red V8. I get more looks and compliments out of this car as it is original and uncommon.
Rig jobs. I saw so many rig-jobs it wasn't even funny. After so many Mustangs, you get a good idea of what and what isn't supposed to be in there. I saw rigged seat belts, radios, engines, air conditioners, fan belts (!), brakes. One car scared the crap out of me because it idled at about 2000+ RPM. When I put it into reverse, it took off and I couldn't stop because the brakes didn't work right. Another car had the floor pans set so far wrong that I hit my head on the ceiling when I got in the car.
Look far away. I found my Mustang 200 miles from my house. Nevermind that it took me 4 weeks to get it running to get it home
(It broke down twice). That's another one. People aren't always honest on what is or is not wrong with it. My carb float stuck and I managed to jam it into 2 gears at the same time since the linkage was bad.
Ask lots of questions on this forum
and sign up with your local Mustang club. Lots of people will help you out.