Not sure on the above, but in relation to "over-budget", I would definitely not go ahead unless I had a pot for contingency.
Lots of differing opinions on how much is enough. Probably depends on the complexity of the build to some extent. Also on the known unknowns, or likelihood of things cropping up. Extensions, older houses, conversions are presumably more risky. Another factor is being able to stick to the plan and not make changes on an ongoing basis, and have reasonable PC sum values that you try to stick to. A good QS is very worthwhile from what I have heard.