AzPete

Any upgrade with aftermarket parts may effect the mpg. Even a set of louder mufflers may make a change because of the way you drive....listening to the sound. I would aim for no mods, but many times owners return them to stock so one may not really know. I know several who drag their 100% stock Mustangs just for the fun of it.

As far as issues, every model has had plenty of issues. Just read up on them and ask the seller if there have been any problems. Yes, water leaked in the early models of the s197, but was mostly cured in 08/09 from what I read. '11 and up have some manual tranny issues, lower control arm issues and such so nothing is safe from issues.

I would look for an 08/09 low mileage if you can take the time. Many little upgrades were done. The garage queen is the best deal....pay a bit more for a slightly used car.