If you're not a pro mountain biker, just get front suspension. I wouldn't spend more than 600-800$ on your first bike. Don't let the guy talk you into a better model bike, especially if you're going to use it 3-4 times a month (at the most).
Disc brakes are a must, some bikes don't put them on, and then you'll need them after. Try stopping without disc breaks when your tired is all muddy. Not happening.
Hit me up with questions.
This is just not true.
A mountain bike is probably the better way to go if you don't want a road bike or a fixie. I would switch the tires out though. Maxxis make a tire called the Hookworm which are 2 1/2" mountain bike tire width, but don't have the aggressive tread of a mtn bike tire. The walls are pretty thick too, so you don't have to constantly sweat over road conditions. Plus, you can go off road in a pinch if you have to. Then just switch out your tires if you want to do actual off road riding.