I would buy a parkwood, build it up cheap with either a new deore set and second hand finishing bits or vice versa, upgrade over time and then swap the frame to a stanton or something similar. Or they do a parkwood build with low end sram for £599 (or did)
Upgrading over time is ok, but unless you dont ride much, the cycle of replacements makes it hard to get everything where you want it to be to swap it onto a decent frame. On that basis, buying complete will usually be better – buy a donor 26er or something for drivetrain and buy a decent frame first.
Also depends what you have in your spares, if you have a lot of parts I would go with the frame first and go from there. Get some rigid forks on to start with.