Find a used MTB with rigid fork, slap some fat tires on it, maybe ask a LBS to tune it, that's about it.
As a buying tip, use common sense, stay away from obvious signs of abuse, rust etc. It's a good idea to get them to undo the stem out of the head-tube and the seat-post out of the seat-tube and check the tubes and the pipes for rust, that's where people usually overlook it.
As for stolen... get a decent u-lock and park it next to bikes with crappy cable locks.
No bike thief will go for the cheap bike with a strong u-lock if they can have their pick of better bikes with worse locks. You said $300 max, if you get a $200 bike do yourself a favor and invest $50-75 of the other hundred in a Kryptonite u-lock.