I agree as much new as possible.

I suggest start with a repop harness and integrate the added systems.

If you are going to do the whole kit-n-kaboodle yourself it can be a painstaking mess.
If you do it all yourself a trick is to take a plywood sheet and nail/tie your old harness to it stretched out like you are tanning a deerskin. Mark the wires and where the connectors are. Then mock up the new harness on the board and leave at least 6" extra wire beyond the pigtail location for extra measure. Leave the connectors off at first and tie the harness with Plastic ties, I use the removable ones.
Test fit the harness and integrate any new components into the harness at this time, make sure you mount the MSD, etc to the car so you are sure it fits to the harness and car.

If it's a simple harness you can finish the ends while it's in the car. If not make sure the connectors fit the harness, mark and trim as necessary. Remove and install the ends.

SOLDER ALL CONNECTIONS That you can. Don't skimp on this step.