get (accurate) insurance quotes on any machines you're interested in - this'll be the biggest sticking point.
Avoid sportsbikes type machines: if its a first / learner bike, you'll almost certainly drop it leading to very expensive damage (fairings aren't cheap, as my boss just found out! More on this later).
At £900, you're not going to get the cream of the crop, so two-strokes are probably best avoided: many will have been previously abused (the RS125 needs rings and liners every 6000 miles, budget a couple of hundred quid, double that if you get it done by a workshop), synthetic 2T oil at £15 / litre. Running it on a shoestring (as many will have been) leads to engine seizure. If you go down the two-stroke route, you best be handy with a spanner. On top of that, some 2T bikes will need restricting to comply with CBT rules (14.6 bhp), and many will have been chuck down the road (ie crashed).
If you want a reliable bike, Honda CG125 or Yamaha YBR125, whilst not being the best looking, are both cheap, insurable and reliable with loads of spares available.
Why to avoid faired (sporty) bikes: I bought a Fazer 600 2 weeks ago, had it parked up at work, and our mechanic accidentally hit it with a van. Result: broken fairing (its a half-faired bike) £323, fairing mount £90, smashed headlamp £151, the usual selection (mirror, indicators, broken footpeg & hanger, bent brake pedal, scraped bar end, rear panel, alternator cover etc). Total bill: £1,246 (yep, you read that right, one thousand, two hundred and forty six quid!). Bloody glad the firm's insurance are footing the bill for that one.....