Question: Worth it to repair bicycle?

 I've been given a mountain bike, it's not in great shape, but I thought it was "serviceable", now I'm not so sure. The tires need to be replaced, it needs a new chain, and a new braking system (the one that's on now has a broken cable). The big issue is the "gear system", the cables are also broken on it, and it won't switch gears. What are some options, I'm getting a bike just for cruising around, but I live in a rural area with not many paved roads, so the mountain bike seemed like the best idea. I just need to know if I'm better off buying a new bicycle, or if it's going to be cost effective to repair this one.