youll be ok on a good bike, id suggest spending about £2500 on a low mileage well looked after WR250f.
it wont need an engine rebuild (all performance 250's are will require a crank and a piston/barrel replate sooner or later).
dont buy an ex race bike.
go for one thats had some fat old bloke green laning on it, it wont be thrashed.
if theyre treated right they can last a long time (oil and filter change every 600 miles), mines now got about 8000 miles on it and is just coming up to the point where il strip the motor and freshen it up a bit.
dont spend £300 on a helmet, it will get trashed....
best advice above all else.....
stay legal (tax and ticket, insurance and mot) and smile and have fun :-)
