Brand new is around £3-£4K from TPS depending on spec and bartering skills. i.e. £4K for a CDL K04 engine, long block with cams and cam belt, chain etc but no ancills. Need to reinstall your own ancilleries, inc turbo, manifolds, alternator etc.
As for used ones, yeah, I can imagine that's a struggle. A lot of them are shagged remapped engines, especially in K04 flavour.
Engines just aren't cheap to do properly. New pistons, new rings, proper factory spec plateau honing, new cams and lifters, new valves, new bearings, oil pump etc (£1500!) head skim, pressure testing, painting.......you're looking at £600ish just for OEM Mahle pistons & rings.
Maybe consider upgrading your current engine with forged pistons and rods, which is probably more cost effective in the long run if you're planning on more boost in the future.