Stick with the klx. It has the most after market parts available and if you break something Kawasaki has parts to fix it. Same with Yamaha and Suzuki but the Kawi is your best bet. Stay away from the off brand crap, they are throw away bikes. No parts to fix them and they will fall apart.
NO
It goes about 10km
it is a pit bike brand that makes pit bike motorcycles and pit quads. they are used in the professional race series
There are a number of very good ATVs suitable for a youth. Some good models one could purchase include '2012 Polaris Outlaw 90', '2011 Pitster Pro FXR 15R' and the 'Mini Motocross Quad'.