I built up a Nashbar mountain bike frame that sounds close to your desired specs. What I learned from it was the following:
Disc brake are nice but really not necessary for street use. Adding a rear rack is now a lot more difficult.
I used a Nashbar rigid carbon fiber fork. A steel fork would have been fine.
My bike has a triple up front and 8 speed rear cassette. I only use the smallest front sprocket on the rare occasions I go off road.
I spent more on the project than if I had bought a good used bike.
The end result was a good street commuter that can be ridden off road. I am pleased with the results, but the money could have been better spent. It was a fun build. Check out the Nashbar website. The frame only cost $80.00. Part of the problem was that a lot of the components I had on hand were in worse shape then I realized. Ride your bike for a month or so, then honestly assess your needs. Have fun!