I'd say if you're in the mood for a project, then go for it. I've done a couple of builder conversions and would consider this one very do-able for a first attempt at a conversion. Do get ready to spend some time banging out the CSS especially if you're not used to either CSS or Builder. Ideally you would build the new theme locally. I wouldn't install and activate Builder THEN try to get it styled to match the old site because it may take you a couple of days. The Theme Test Drive
plugin might also work. I've used it with some success.
However, if you don't plan on changing the framework that much Flexx should be more than adequate. That's why I said if you're in the mood for a project...