If you are considering smaller bikes, you might still want to consider a Chinese pit bike. I think the way they are designed and used, pit bikes seem to hold up quite well. They are much lower cost than full-size Chinese dirt bikes, such as the Apollo 150. I think the Chinese pit bikes will make a near-perfect 'learning' bike. Start with Taotao's DB17 and go from there.
http://taotao.us/index.cfm/dirt-bikes/ Apollo and a Coolster put out some low-cost, decent quality pit bikes in the 125cc range. The distributor for the Hawk 250 enduro carry Apollo and Xmoto bikes as well.
http://www.therps.net/# That being said, if you can get a reliable used TTR125 for the same cost as one of the Chinese bikes, go for it.