They can be very reliable, especially if stock. But you need to check the common problems out before you buy so you know what you are getting into.
Some items to check:
1. Does the rear end whine?
2. Does the traction control turn off and on or is it stuck off? If so, the ABS module is bad and they don't come cheap.
3. Open the trunk and check the channels for any rust. Those Mustangs have problems with rust on the side channels at all four corners, so be sure to look good.
4. Pop out the trunk carpet and check UNDER the spare. If there is rust, that's a sign that the trunk weatherstrip seal failed at one point. Make sure to check the condition of the weatherstrip as it is prone to fail.
5. Check for an aftermarket radio. If it is still factory, make sure it works. Also check the window speakers if is is a Mach 460 system as those tweeters go out first.
6. Check the power locks/windows/mirrors
7. Check the horn
8. Make sure it has a clean title
9. Check the black plastic hood cowl and see if it is cracked/broken. This is the piece closest to the windshield and is black on every car.
10. Turn the A/C on high for a minute and check to see if it is blowing small, greasy foam-like particles. If so, the insulation on the A/C piping is going out. It makes a heck of a mess on the seats, clothes, etc.
11. If it is modified, ask what is done. If it is an intake and exhaust, that's fine. If it is geared aftermarket, ask who did it. If the buyer did it, walk. Almost NO ONE can install gears on the New Edge Mustangs right without proper tools and experience.
12. If it is a convertible, check that the top works. The hydraulic fluid pump for those is under the back seat, so save yourself a headache and make sure it is working ok.
13. Check the third brake light for fading/cracking
14. Check the headlights for yellowing
15. Check to see of the steering wheel is straight at highway speed or if it is off to one side. These cars are prone to steering rack/steering column issues and they are expensive and difficult fixes.
That should get you started. I would still recommend a pro inspection if all that checks out, but those are easy things to look for. Some are livable - you can buy new headlights. Some problems you don't want - those ABS modules average over a grand these days IF you can find them. And some are deal breakers - rust around the trunk channels - walk away.
Hope this helps some.