I would say all new wire harnesses is the way to go vs. trying to salvage the old. Keep the old harnesses to sell with the used equipment (albeit cut off somewhere).
New tested harness really is the best and fastest way to go. Since you are not comfortable with that sort of thing I would suggest you have either Stein Air or Park Rapids aviation (or similar) make up your harnesses. When the new equipment arrives I would mount the items and measure the systems runs for the wiring. Send your measurements with the connector kits that came with the avionics to the harness maker. Advise the harnessing contractor whehter or not you included service loops in your measurements. I find it best to just give the nominal lengths and let the harness guy add a service loop on his own.