Check the coverage, of course, but pay special attention to electrical type stuff. Crazy things like seat warmers, power mirror motors, door latch and lock mechanisms. I would not worry a bit over drive train components myself. $2500 can add up quite fast on things like a dash coming out to replace an evaporater core, a fuel pump, or even a stone ruining an A/C condenser.
If that Mustang is loaded with plenty of optional equipment, I would certainly consider it. If it is transferable it might very well sweaten the deal if you trade early (you may well eat it too). I'm not familiar with the newest generation of Mustangs, but my past and current experiences convince me that while fun to drive they are a bit cheaply designed and constructed.
For me 100k ain't many miles though. Roughly 5 years of driving, and I'm pretty easy on my gear. I would either pass on the coverage, or sell the car had it already proven problematic. 100k around town is far different than in a rural setting though. If you find that you traditionally have car repairs, I'm betting you will continue that trend, if however you rarely have trouble that too should continue. I am an optimist as far as cars go, I very rarely have troubles.