This might have more to do with the particular example of the Mustang that you happen to choose than anything else.
I have purchased cars off of Craigslist with one foot in the grave. I can say from first hand experience that there are some cars that can be a good value to fix up. There are others that even if FREE are not good buys.
Mustangs also tend to be heavily modified. Some modifications are better than others. Some owners are better modifiers than others.
If you don't plan on doing the work yourself, then choose a better example and stay out of the shop all together.
If you know what you are doing, you can keep a solid Mustang running for a very long time. My 2000 GT is at 230K miles and climbing.
Be sure to look over any car you buy from any location. If you do not have the ability, take it to a mechanic. I bought an 02 that had been lowered with quality parts.....rode like it had solid tires...thought the shocks were bad based on po. Other mods were done so so. The 00 I bought in 07 was sweet....just a couple of paint issues. The 02 I have now is great mechanically too, just had a fender replacement done pretty good prior to my purchasing. All were found on CL. More records the seller has, the better. Check for proper tranny operation and clutch if manual. I would not buy one with any issues that you would have to repair.....unless the price is real good.
You will find it to be somewhat of a bother in stop and go traffic but not a real issue once you are used to it. You can always put it in neutral and release the clutch at long lights and such and then pay attention to the light changing process so you can slip it back in gear and be ready when it is time to move. Good luck with it.
my car off Craigs list , Mons car for sale never been in the snow and is pure stock. in mich. rust is a thought. this car is exactly what the ad said. I looked at 6 others that were not worth buying. good luck ,I love my 96 GT.