Trust me, if you spend that money on a new bike, no matter what it is, it will be downright dangerous to ride on advanced trails. Even with the good brands, the entry level bikes are only meant for moderate riding, and you really need to spend at least 400 lbs for a new bike that will begin to hold up and be able to handle the trails. You'll need to buy something used if you want it to hold up (and have patience, because even used it's hard to find something good for this price).
Either way, go with a hard-tail, because a good full suspension won't happen at this price, used or new, and cheap suspensions, aside from breaking, will be heavier, and sloppy (soaking up your energy and giving in too much). I have a hard-tail and do advanced trails without an issue, keeping up with the soft-tails (except those amazing riders that cream everyone). It just means I bounce around more than the suspension bikes and I need to use my legs a little more, but hey - it's a sport right? And a good hard-tail will keep going long after you've walked your cheap and broken suspension bike home.
The front fork, though, is a good idea to help with the ride and it's easier to find a bike with a decent one since they've become such a standard piece of equipment on the bikes. But again, if you go for a bike with a low-end fork it will have limited travel and will be rather soft and sloppy, with very limited, if any, adjustment. This means it would more likely get in the way of your riding than actually help. I'd rather a rigid fork than that.
Another thing you need to think about is disc brakes. For wet and muddy conditions discs will keep working reliably, and if you plan on doing any downhill runs they'll stand much more heat as well. Hydraulics are optimum for strength and reliability. Cable discs are fine, but you'll constantly need to be re-adjusting them and they don't have the braking power of other brakes. If most of your riding will be in dry conditions and you plan on doing mostly flatter trails, V-brakes are actually incredibly strong, and are reliable and easy to work with. Older bikes are more likely to come with these.
Don't fall for cheap bikes that look pretty. I've seen the department store bikes with disc brakes and full suspensions and to the untrained eye everything it needs to conquer a mountain. All for just a hundred quid. Looks cool, but nothing actually works properly, and none of it will hold up long, especially on trail riding. Don't buy at a department store, period.
Last, make darn sure you get it sized right. The only thing that will ruin your ride more than a lousy bike is a bike that's too big. Stand over the frame with both feet flat on the ground, and you should have about 5 to 10 cm of clearance over the top bar. If not, it's too big - don't buy it!
