Second hand is the way, you'll probably find something like a complete On-One 456 for that. Be patient and keep your eyes peeled on the forum, they go fast!
New prices are prohibitive, but there are many of us on here that fettle away chopping and changing on a regular basis.
Sounds like you're after an XC bike but think a little longer term. Something a little more solid may mean you can ride more challenging stuff as you progress. Don't get hung up on weight and concentrate on hardtails for your budget. They're great, but you'll hear FS only snobs saying nothing compares
Let us know when you find what you're after and we can steer you and your money in the right direction. There's a lot to cover, so it's a little difficult to say. That being said, you'll definitely be after either Marzocchi or Rock Shox forks. There are lots of bikes at £500 which are let down by rubbish forkage.
Also don't blow your beans on something expensive before you know what you want, a lot of us are riding a Trigger's Broom.
You mentioned £750, £600 for a second hand complete bike will get you something really good. It's a potentially very expensive sport, don't throw your money away!