With what you're looking at replacing, buy the complete bike, the things you're looking at replacing aren't that expensive or complicated to replace. You're keeping the major components. You could replace all those parts easily yourself, if you don't have bike experience, doing the Headset and BB might not be easy/cheap for you.

I personally, almost wish I had gone with a frameset and built it, but I'm glad I don't have to mess with the BB or headset as those seem to be the most complicated components. I've replaced the wheelset, rear cog, seat, pedals/straps, chain, and bartape so far. Looking at replacing the Stem and bars next month.