This is a good thread. It offers beginners insight about what the costs really are to build a reliable MB.
For $1000 anyone should be able to build a dependable MB, but $350, no. That doesn't mean you can't build a $350 MB, and upgrade the components over time. But you'll probably be close to $1000 by the time you reach a level of dependability where you can ride all day without worrying about breaking down 20 miles from home.
Personally I wouldn't want an expensive bicycle as a base for a MB, because I like to choose my own components. When choosing a new bicycle I'm mostly just looking at the frame. It has to have the look I want. Just about everything else is irrelevant to me, because I'm probably going to change it anyway.
There's only 5 parts left on my bike that are original. The frame, seat post, seat post clamp, rear fender, and peddle chain. If you were to duplicate my bike, it would cost right at $1000.