hmmm, well you've got quite a few options, and a good budget! But you need to sus out if your gonna go 2 stroke or 4!
Go 4 stroke if your gonna be using it to ride long distances a lot, going to work back and forth and such. Plus 4 strokes are reliable! But slower and some might say a bit less fun than a 2 stroke :P examples of nice 4 stroke crossers would be :: Honda XLR125 or XR125, Suzuki DR125, Yamaha XT125
Go 2 stroke if you want something thats gonna shift for a 125, 2 strokes are faster and a bit more fun but slighty less reliable and you have to buy 2 stroke oil for them a lot. Good 2 stroke examples would be :: Yamaha DT125R , Honda CRM125, Aprilia RX125 or ETX125 , Kawasaki KDX125 or KLX125 or KMX125, Suzuki TS125R. Dont get a derbi though as they tend to fall apart, and dont get the aprilia rx125 unless you have cash, lovely bikes but if they go wrong, expensive parts!
Hope this is some help chap, if you want any more tips im :: roozor@gmail.com
overall winners in my eyes, if your going to get a 4 stoke GET a honda XLR125.
2 stroke on the other hand, i'd say DT125R or KMX125.
Alex
ps :: just about to buy my new bike B)