hey man, I am 32 yrs old, 6'0" and weigh 205 lbs. I am in pretty good shape. Last year I bought a '05 YZ 250. Fun bike, very quick, out in the open. As a beginner its pretty tricky on a tighter set up MX track. I don't get to ride alot, maybe 2-4 times per month, so that lengthens my learning curve. I went with the 2-stroke because its easier to work on, cheaper to work on and cheaper to purchase. However, as a beginner, on MX tracks, I really can't reccommend a YZ 250, cause I think I am pretty tough and that thing gets on top of me too much, its a little too bad a$$ for a starter bike, if you ask me, who is a starter and has one. In fact, it got away from me last week, hit a fence and broke off the foot brake pedal.......anyway, I am kinda shopping for a good condition 125 2-stroke to hone my skills on, and keep my 250 for open riding and when I am a better track rider.
There are many, many 2-strokes for sale, the good part is its a buyers market and you can get some good deals, bad part is it prob. will be even harder to sell a 2-stroke a few more years down the road.
Of course, because of the maintenance, that Should be involved, I would be afraid to buy a used 4-stroke MX bike unless I knew the seller VERY WELL.
Anyway, I am only a yearling beginner, so you can take my experience for what its worth.
rtb