…and if you do, whats your bike set-up? Looking at building one up, and considering options. Frame choice, forks and gearing (1×9,1×5,2×9???) are main things im unsure of.

I use a dialled bikes holeshot. Forks aren’t too critical really as you’ll be running them fairly firm, people use everything from coil pikes to air fox forks. You don’t need more than 100mm, personally I’m currently using some air 66s reduced to 90mm. Gearing is almost always 1×9, chain device, road mech and road cassette. I use a 34t ring and 12-23 cassette for racing.

Im thinking for frame size that possibly an undersized ‘core hardtail may do the trick (maybe an inch or two small), or would you suggest something with a tighter back end, something specific for the job?
Also i have some F120’s, reduced to 100mm theyll be good ?
(sorry complete 4x noob)

there are a few specific frames think yeti possibly produce the best, but there expensive.

If you are a noob just cobble something together cheap, the bike isn’t too critical. Anything small framed with some very fast rolling tyres is fine.