What do I need for a proper under-dash rewire?

Year One has the harness for $519, discount eligible, with a 30% discount. That is about $363. I have found that the price varies wildly on some of the more expensive stuff, so you need to check a bunch of sources. Probably still a lot more than you can do it yourself for, but it does come with a new fuse box which is nice, as water leaking from the windshield wiper grommets usually kills the fuse box.

Make sure you replace connectors on the mating harnesses too and use dielectric grease. The engine bay harness may also be a good one to replace also.