the two smokers are fun, but can be a handful. currently i am doing courier work on an nsr 150 sp, its a blast. but you have to drop down three gears on every hill to get into band, blah blah, yarda yarda. it rattles, it buzzes, it blows smoke when you give it some after its idled for a minute or so...
fourstroke...if you cant ride(ha ha), or like a nice smooth power delivery with some low end torque...
two stroke, if you can ride and just love ripping through the gears, making chainsaw noises
pw50 etc may be good for a learner, but a kx65 is not! or even a ktm50...
80s are just about right. 125s can be nasty, and 250s will chew you up and spit you out...but are so much fun!
neighbour bought a kx65 for his son a few years ago. i had a go, and sort of forgot that i had to slow down just as i was getting some speed up, dropped it.
one tap of the rear brakes and it just slid out from under me. they really are fun, but find a big wheeler, or an 80. the little wheel 65 is just too twitchy to ride easily.
on that note, the neighbours one seized up after maybe a week. you never know whats been done to it by previous owners(if its been raced...dont touch it), its really easy to damage the cylinder when things go wrong, and parts can be expensive (my nsr150, i bought a complete bike for the same amount as a new cylinder...and to get a cylinder re-plated, much the same cost) whereas a china bike, the engines are cheap, you kill one, its swapped over in ten minutes, who cares? but bits fall off all the time!
you gotta keep an eye on the oil, if the pump fails you are screwed, and you cant just shove old cheap lawnmower oil in them. (unlike the pw50...)