Yamaha makes some of the best 4 strokes on the market, I have a 07 yz250f and it's extreemly fast and has sick tourque, sometimes I can sit on the tank and not keep it down and i'm 6'3 185 pounds so keep that in mind if you're not an experianced rider it may be to fast for you. A cheaper route would be finding a 125 2 stroke, most of these are in abundance anywhere and can be easily rebuilt season after season or depending on your riding 2-4 times a year. If you do look for a 2 stroke that's somewhat older(early 90-now) make sure to ask if it's got a fresh top end, if the forks have been rebuilt and look around the bike to see if it was taken care of. I've gone to look at some that where beat up and gotten them a lot cheaper than what they were asking for it so it can help. Plus they have so many upgrades for them it's sick. Honda crf's and suzuki rmz's are also great bikes to take a look at. The honda crf150 is a pretty nice bike and great for beginners- itermediate riders. Look around and if you find a bike you like then go for it. Be wary of the older yz250f's from like 2001-2005 if they've been riden a lot they may need work that even the owner doesn't know about, 4 strokes arn't bullet proof, they do need new top ends in them which is more expencive than a 2 stroke, because you have valves and springs and cams that all need replacing after a while.
