Got a dmr rhythm frame, rigid onza forks and some halo wheels

What else should I be looking at or avoiding?

Balls.

Avoid – fence posts, nettles, brambles, holly bushes.

Plastic pedals and decent riding shoes – but not 5-10s find them far too grippy – nike 6.0 are spot on imo

Bars and stem? Obviously ill need em but should they follow today’s standards of wide bars / short stem or will my old 660mm medium rise bars / 90mm stem be ok?

Single speed or 9speed?

Balls are more difficult to find than cheap mtb gear!

Narrow bars, lots of rise and a tiny stem is ‘normal’, ditto SS, in general you want as few bit’s on it as possible as at some point you’ll crash and won’t want brakes/mechs/whatever impaling you.

Cheers – makes sense when you think about it

Now about the balls…?

Singlespeed as you will smash a lot of rear mechs crashing.
90 mm stem may be a bit too long but you can always change it later if it is.
660 bars will be fine, I used to run mine so narrow that I had to modify the brake lever to clamp onto the bend.
Strong cranks are a must, you will break them, three piece bmx cranks are heavy but do the job nicely.
Small narrow seat angle back a bit.