If you're gonna do it, do it right
Come... Join us.
I gotta go with the majority here... Unless you plan on modding a V6, I think you'll find it disappointing. The power just isn't there, that sound isn't there (...honestly, I hear the growl in my dreams), and the value will be lower on down the road. I got hands on a sheet with the Mustang Production Mix from '05 or '06, and it was at least 65% V6's.... and that's all that was left on the lots late in the year.
If you seriously drive a mild 55-60 mph all the time, you will probably have no problem getting 24MPG+. I'm getting around 18-19 on avg, but that's driving through downtown L.A. at rush hour twice a day. Anytime I can keep it moving and stay below 70mph, I get 23-24
And while most go for power, it is possible to have it tuned for efficiency's sake (blasphemy, I know.. it's like breast reductions for health reasons

) instead. You could also get your gears changed to make it more efficient. In fact, with all the things we people do to our machines to get more power, a lot of the same things can be done in a different way to make it less thirsty.
Worse comes to worst, spend another $1k-$2k on an old motorcycle and use that for your commuting, use the car for fun times.
MOST IMPORTANT: Ask yourself, why do you really want a MUSTANG?
-wwar1ace