Water pressure problems--old house and new on demand water heater

We are on a well in hard water country. We have recently replaced our old gas water heater with an on-demand heater (which is feeling like a big mistake)and done some kitchen and bath remodelling. OUr problem is having pressure to activate the hot water heater. Everytime a plumber turns off the water supply to work on this problem or that, or lately install a new vanity and sink, the fixtures get clogged when we turn the water back on and we don't have enough pressure to activate the hot water heater. We have considered an in line filter to the bathroom line, and my plumber suggests I replace my brand new sink faucet with something less clog-able. Love some general advice--and perhaps a tip on the best faucets for plumbing that may experience some rust and sand and lime build-up.