In My humble opinion, you dont need power offroad, what you need is a bullet proof bike, as you will bin it.....but that is part of the fun.
I started (and still run) a Drz400. It is fine for me, cheap, rubust, and pretty low maintaiance
The must thing to do / buy:
1) Body Armour
2) Knee Pads
3) Join the TRF
4) DO NOT USE ILLEGAL LANES
5) Again DO NOT USE ILLEGAL LANES
6) For clothing, I have army Combats & army gortex waterproof trousers
7) Waterproof socks
8) Remember if you want to race, go to an MX / enduro event/track.. much more fun
You could do a Trial day, for example at Trail world
http://www.trailworld.co.uk/ or Suzuki
http://www.wheeldontwo.co.uk/That way you get a taster without shelling out a load of money.