Lots of question marks. I've been riding for 40 some years and have never broken a chain or a cable.
You will have to elaborate about the wheels. Is the problem with the hubs or the spoke tension?
The 6400 RD should work fine for 6/7 speed for sure, and probably for 8 speed. If it's not allowing shifts to cogs on either end of the cassette, it's probably because the limit screws need adjusting.
I converted a later road frame to flat bar, thinking it might be nice for easy day rides. It's one of my least favorite builds. I have a lot of wrist pain when riding in that position with little option to change hand positions. That said, flat bar may work fine for you depending on the length of your rides. If you go that route, what type of shifters do you want to use? I'm not a big fan of Rapidfire, much prefer the thumbshifters. The cable adjusters for the down tube shifter bosses are easy to find, just another $15-20 item to add to the project.