hmmm, you sound like a good candidate for a 125... they are wicked accelerators, you don't wiegh so much that it won't go, and they are a little bit smaller than the 250's.
I would not suggest to most that they start learing on a MX bike, but if you have a lot of street experience, and aren't easily scared off, you should get a grip on the basics pretty fast (a lot of the initial learning has more to do with clutch/throttle operation, which is essentialy the same on a street ride). I would never recommend (again) that someone start with a 250 MX'er, no matter how much balls they have... the bikes are unruly fast, pretty tall, and geared for nothing but GO.... that coupled with the extra power (which no one needs while learning) make them very frustrating to learn on.
If you get a used 125 for a good deal you can sell it later and hop on a 250 if you out grow the 125 (which won't be too quickly). I don't think you will be let down by a 125 2 stroke MX'ers power. The only thing you may want more power for would be desert dune riding and serious hill climbs...
and, yes, you are going to F*^% it up anyways... but sometimes with a used machine you spend all your time chasing problems and very little riding.... it can be very frustrating....
find a sweet deal on a 125 in your area... get the least hashed out bike you can for your cash (pay more attention to the shape it's in than what brand it is!).
oh, and get the proper gear!