Well, for a good MTB, at least how much I know about it, you just need strong, double alloy wheels with good chunky tires, shocks on the front are absolute must, the middle/rear one is optional, but I do without it, alloy or carbon fiber low frame, and disc brakes..
In my country, for that kind of money (500 pounds, which is about 600€ ) you could assemble by yourself a pretty good and durable bike..
There are some MTB and downhill trails in my place, some of them are very steep and with rocks, my bike is similar to the one I described (except the disc brakes) and the ride isn't always smooth, the middle/rear shock (or absence of one) does wonders to your behind.. ;)
At the moment I am using city slick tires, so no downhill runs for now.. ;)
I hope I helped.. ;)
EDIT: I just checked out the bike in your first post, and that is about what you need.. ;)