Older rigid or front suspension mountain bikes can make great bikes for commuting and general transportation. Strong wheels and frame can readily cope with rough ground or a few potholes. A simple change of tyres can adapt the bike for whatever conditions you're going to use it for. Most of the time, I'd recommend tyres that are smooth in the center of the tread, with knobs on the sides which come into play in rough terrain. I use Schwalbe land cruisers for combination road/trail use on one of my bikes and find them well suited. If you find you're going to be riding a longer distance on the road, throw on a pair of fat slicks.
I'd definately recommend looking at used bikes for a first bike purchase. Your first bike will almost never be exactly what you want, but will give you an idea of what you do want. You can then go and spend a bit more money on a second bike.