Racing is 90% rider 10% bike.
Don't let anyone tell you that you have a disadvantage on a 125. Until you are in the 125A bracket, a 125 will suit you fine.
A YZ125 is a great choice. As far as brand popularity goes Honda and Yamaha are the 2 most represented brands at most MX tracks (around here anyway).
I understand you want a brand new bike, but I would seriously consider getting a slightly used one. 2 strokes haven't really changed much at all in the past 3-4 years. You can pickup a 2004 YZ125 + a full set of good riding gear for the price of a brand new one.
It's not hard to find some guy who rode his YZ 3-4 times, then gave it up because he got married or something like that. Or even find a previously raced bike thats all decked out with after market stuff.
The differences between a 2004 model and a 2008 model are minimal and would probably only be noticed by very experienced riders.
Tell BoD (Bank of Dad) that you want a 2004ish bike, plus $800 bucks for a helmet, roost guard, boots, gloves, pants, shirt, googles. Then take $50 and pickup a new air filter, 2-stroke oil, crankcase oil, and some chain lube. Buy the bike and do all of the regular routine maintenance before you ride it.
Voila you have a bike that is basically good as new and much cheaper.