Kawasaki's are great moto-x bikes, had lots of them over the years, started on a KX80, then a KX100 big wheel, KX125 and finished up with a KX250, apart from drinking fuel and using quality 2 stroke oil, you should be OK, look out for nackered piston rings as this is the most common issue, I used to get mine rebuilt at least once per year (usually twice with a seasons racing/practice), I like the 4 strokes, but you cannot beat the sound or "Powerband" of a 2 stroke
A KX125 is a good bike to start with, but if your a bigger guy I would say go for the 250 as otherwise the 125 engine might struggle with your weight (This is the reason I went from a 125cc to 250cc, going from a skinny 17 year old to a grown 19 year old makes a difference
Regards injuries I was lucky, only a broken finger twice and a broken toe once (practicing in trainers!
) but my mates had plenty of injuries, wrists, legs and collarbones are most common
Get a good pair of boots that fit well and are comfy they make a big difference