There are loads of good 125 motocross bikes road reg'd. Stick to jap bikes as the build quality is about 1 million times better. I had 2 brand new KTM 250 SX's and they were both the worst bikes I've ever owned. They are made to be fully stripped after every single ride and take a lot cash and time to look after, and even then, they wear out really fast compared to any jap bike.
My first 2002 KTM blew up in the first 2 hours of riding from some one in the factory leaving the gudgeon clip loose on the little end, and then I was with out a bike for 5 months until KTM finally managed to aquire the parts under warranty. The list of faults continued with that bike, so KTM gave me a new bike in 2003. The new bike was even worse! That was a complete new chassis and rode like a cammel. It was quick, but whatever I did with the suspension, it would never ride right and couldn't put the power down. When it was brand new, there was loads of play side to side in the swingarm. KTM said they had over machined a number of engine cases (mine being one of the bikes) and gave me some crappy washers to space it out. They called this a mod lol. They lasted half an hour then were chewed up. In it's first month, the seal in the clutch slave cylinder popped whilst I was flat out coming out of a burm in 3rd. The bike wheelied so I went to feather it on the clutch, and the bloody lever just went straight into the handle bars (spewing fluid into gearbox) and flipped me and the bike big time. I did both collar bones, some ribs and lost some discs in my kneck. The bike was wrecked, it needed a new sub frame, the seat was snapped in half, exhaust, air box, rear tail unit, bars, and forks were slightly bent. The KTM dealer I used were always getting them back for loads of bits falling off etc. KTM were quite quick in reapairing this bike as I was injured from it, but I was still out of play for a while from a piece of sh*t bike!!
I know loads of people who have had bad KTM's, and I don't know of any one who has been satisfied with their KTM motocross bike. I know a lot of enduro lads sware by their EXC's, but then they don't get ridden as hard and have many different parts from MX bikes
Sorry to rant on about it, but the word KTM just gets me angry