First thing you need to do is discuss the problem with your condo association. Most of condos are under very strict rules on what can and can not be done. and by whom it can be done. If you were in a private single family home, then the ball would be in your court. You have discovered one of the big problems in retrofitting a tankless water heater. Whether gas or electric, they require more power than homes are normally equipped with. There are other negative factors, but that's not part of your question. As far as the material to repipe with, again you condo association may have some say in this, but Pex is likely the best option as it would require less demolition of walls, but, listen to the plumbers. They know the water conditions in your area and they can determine if a flexible pipe like Pex is better for your situation than a ridge material. In my opinion, this is a one-time-do-it-all-at-once job.