Is it an automatic? Remote start is possible on any car, it's just not very safe on some(cars with no neutral safety switch or manual tranny). As the mustang more than likely doesn't have a neutral safety switch if it has the original tranny you'd have to put the neutral sense wire to ground and pray you never try to start it with it in gear.
Any alarm from DEI is a safe bet, Viper, Clifford and such. Some swear by compustar, but I've never been particularly fond of them. You can find decent deals for clifford RSX3.5 on ebay which is two way remote start alarm, and it's a pretty straight forward inistall. A little time consuming , but if you take your time and know how to use a multimeter it's pretty easy.
In my early days of installing before listening to the warnings of not installing anything with the battery connected I was installing an alpine 8080a alarm with remote start in brand new 560sl benz. I hadn't gotten around to connecting the neutral safety switch wire as it was a b**** to find on that car. I got up for a few minutes and one of the other techs got into the car fidgeting under the dash installing a k40 radar detector. Well I wasn't done and had a few bare wires, one specifically was the remote start trigger. The car was in D1 so we could have some extra room to work. The next thing I hear is the benz trying to start, the tech squeeling like a pig, then CRUNCH, BANG. It went about 50 feet across the garage had managed to start and ended up embedding itself under a brand new F350 econoline van setting off the airbag and breaking two of the techs ribs because he was wedged under the dash.