What kind of riding will you do? How often? I go infrequently - sometimes with my kids - and the lower end bike was fine (around $260 - Specialized Hardrock). I do fairly easy stuff, some moderate single track, lots of gravel roads. St Edwards Park, Lord Hill, Iron Horse trail, Mackworth park, etc.
I recommend getting a bike with the shock absorber on the front - but that's just one step above minimal.
I don't have toe clips like I do for my road bike. That'll cost you a bit more.