You already have the motor kit at 150£.
Now search for the rails, at least the supported round rails (or hiwin rails if you want to cut metal with precision). Add 6 rails at your desired length plus 12 linear bearings.
Add 3 screws with nuts, ens support and coupling. If you want precision look at the ballscrews instead of the trapezoidal ones.
Add the cheapest spindle you can find, some sensors or switches, relays and of course all the cables.

If you still are under 500£, which I really doubt, add some aluminium or steel as the other components cannot levitate.