Pex is a good choice. I'd use 'shark bite' fittings, then you can mate the 3/4 copper directly to the pex piping. Allow for expansion in the pex, hook it up, bleed it, enjoy. Don't forget to get the removal tool for the 'shark bite' fittings, so that you can take them apart when you need to. THEY CAN BE REUSED MANY TIMES, or remain as permanent. The fittings aren't cheap, but you can hook hem up in seconds and get back to the kitchen remodel. I'd get that ball valve on the tankless coil and look over the expansion tank issue. Is it a newer pressurized (xtrol) one or the older solid tank, less rubber ballast type? Most big box stores have the fittings, and all supply houses, too. Some hardware stores are catching on too. Instant results, no soldering, no leaks,......no brainer!