I am with tsl, the stuff you want to change about a mtb is going to jump the build price up to the cost of a secound hand bike that is closer to what you want. I would recomend a cyclecross bike, I have seen used Surlys go for the 500 to 800 range for a really nice bike and they are bullet proof. Mine is my commuter and before that I used it as a light duty mtb.

As far as the disc brakes go...........look into Avid shorty 4's. I have discs on my mtb and the shortys on my CX commuter and the avids are almost as good in my opinion.