Defo 125 to start. Bikes at that price are money pits tho. You really need all the gear and buy the best you can afford cos crashing hurts.
I've got a 07 Honda CRF450 the now and love it. Much easier than a 2 stroke to ride.

If you're bored tomorrow get down to Duns and watch the British championship. Its the first time in scotland in years and its a awesome track!