I'd recommend a Laser Trident 2 w/12x10 widescan coil(UK version of the Tesoro Vaquero). Should come in quite a bit less than £650. You'd have a deep machine with very fast recovery speed and it's pretty easy to learn.
I'd have to go with fella's position.....I've got a Fisher F75 it's a more complex machine.....I'd lean toward the Trident he mentioned...starting you want what's called a beep and dig machine with an easy learning curve.
The F75 might frustrate you to no end starting out.
Don't underestimate what a solid basic machine can find....my everday machine is a Tesoro DeLeon.....I guess the US
version of the UK Trident.....and I do ok....
Oh well...In the mean time you can start bulking up the upper body. You're gunna need it to swing that thing for more than half an hour at a time.
Good luck!