Ooohh, so now we have a goodie two shoes user entering the mtb world, great first post. It's been a while (actually, I only come occasionally and only see the newbie's aol'ing it up. Sorry to the rest of you goodie-two shoe newbies.
Just kidding.)
As for using MTB on streets - go for it. Speed (with right tires), strength and price, you dont have to pay much to get a reliable mtb. 300 can get you a pretty decent workhorse
. A little extra money paid to a bike shop goes a lot farther than the little less you pay for a department store bike. I'm not assuming that you are going to get a department store bike, but it's a mistake I made, and I had to pay for.