Can vary,,depends if brand new or used kit.
New can cost upto £5k fitted and mapped which includes, turbo (small one greedy and similar only good for upto 170-190hp), fmic, manifolds and downpipe, mega squirt ecu etc.
Second hand kits come up vary rarely about £1-£2k

You can fit your own chosen turbo though like wrx td04/05 etc just need to fabricate manifold and downpipe. This Is what I was gonna do but after extensive reading I think I'm gonna go for a garret gt25 (mk1 focus rs turbo) which seem to give good power and nice spool, good for 250hp in a mx5.

There's a guy running a gt30 at 400hp

My advise would be if your wanting one is get one with LSD mostly found on imports and special editions as mine is track focused and not drifting as you would want it for, but it can skip out on corners and then the LSD works it way round (scary in wet)

Also a cheap fun drift toy if you want,

Again its how you set them up for how you want to use it, get it set up for track with all the relevant mods and some 888s and it grips,so all this talk of its only good for drifting is not true, yes most people buy them for drifting so don't bother with the ones with LSD or weld it or remove it and have cheap **** tyres that are easily skippy.
All parts are interchangeable and there's a wide parts choice out there at reasonable prices for what you want to achive.

P.s you can also supercharge if you wish too, personal prefrance on power delivery

Join the hairdresser bumboy brigade and talk bum and cars

Saxo vts and 106 gti are also good handling cars especially with a carb set up, but again fwd and French reliability. It's your choice.

Maybe an old m3 stripped out and some slicks is a good capable toy too.

Good luck on your choice

P.s if you want to test/drive it, I'm sure I'll let you