the bike in general doesn't make the brake good/bad if you have £900 to play with you could build a very nice bike to start on, since you've ridden bmx so long it'd make sence to stick with 20" wheels, unless your a big guy, have you found your local riders?
spec'd this up:
£923, thats with tarty prices, luckily they offer a price compare so if you shop around for some bits (like the BB7s ) they'll be cheaper.
Echo TR Rear Disc 19": Black Hub, Black Rim
@ £99.00
Monty Eagle Claw 19''
@ £29.00
Kenda 19'' DH
@ £7.50
Maxxis Welterweight (Boxed) 20''
@ £5.00
Atomz Rim Tape 19/20": 19", Black
@ £2.50
Atomz Rim Tape 19/20": 20", Black
@ £2.50
Trialtech Sport Screw On: 14T
@ £15.00
Onza Comp 2010: Complete with chain tensioners
@ £179.00
Echo TR: 170mm x 30°
@ £30.00
Onza ISIS with bolts
@ £35.00
Trialtech Sport Forged: 160mm ISIS, Black
@ £50.00
Tensile 60 Click: Black & Bronze
@ £40.00
Trialtech Sport Riser Bar: Black
@ £49.00
Trialtech Lite Z610 HX, 3/32''
@ £14.00
Avid BB7 '09: 160mm (Fits front and rear)
@ £120.00
Echo TR Front 20": Black Hub, Black Rim
@ £79.00
Neon Conquer '09: Silver, Disc Only
@ £69.99
Maxxis Creepy Crawler Front 20''
@ £16.00
Onza Bashplate with Bolts
@ £18.00
TartyBikes Bashplate Rubber
@ £4.00
Trialtech Race 'VP': Black
@ £16.00
Because V-Brake Lever
@ £20.00
Clarks Brake Cable Kit: Universal kit
@ £9.00
Monty Foam
@ £5.00
First Blog Z
@ £9.00
whatever you choose try and have isis cranks/bottom bracket, sealed hubs (front isn't as important as rear), high engagement freewheel.
with brakes maguras are popular, but cable brakes can be very powerful, the BB7 disc brakes are monstrous peices of kit, even the 160mm rotor versions have alot of power!
otherwise buy an onza master.