I have 2 years of street riding, with at least 10 track days under my belt. I'm still in the slow group, but i was on the verge of moving up before the season ended. I've switched off between my f4i and rc51, so I'd like to think i have decent bike control. I want to get a dirtbike, preferably used, but have no clue where to start. Should I get a 2 stroke? 4? 125? 250? Anything I should look out for?