IMO, starting MX with a 250cc 2-stroke may not be a good idea if you don't have enough off-road experiance, it's the same with YZ400.
I'm 63kgs like you, and I'm a beginner too. I'll buy KX125 tomorrow from a friend. If you think about riding a lot in trails, get a 4-stroke. If your priority is learning MX well, get a 2-stroke.
I recommend CRF250X as a trail bike(it works well on track too), and YZ125 as MX bike. But anyway there are slight differences between brands, so you can get other bikes too, of course.. As I say in all posts, I don't know anything about prices, so if I'm telling more than you can get, sorry..