There have been other threads pertaining to this. A couple of builders said that it would have been easier to rip everything out and start over. If you do that, it would be plug and play (using Van's SkyView harness).
If you want to reuse existing wires and pull in more where required, it is doable but will require a lot more thinking, and perhaps frustration.
I've looked in to buying 2012 to 2013 or earlier RV-12's with the D-180 systems, and based on wanting to fly, not wrench on the plane or hassle with R&R and paying someone else to do it, just figured that since I fly out of a class D airport here in So CA, the added expense to update and the headaches was not worth it, I swould just buy a newer plane and pay the price of it installed up front. Not worth the headaches and trouble shooting if you just want to fly.
A very nice 2015 RV-12 N16VW, with a Skyview Touch, white with blue paint, 2 dim auto pilot, with about 75 hours, just sold last week down in Corona, CA for $60,000. It sold in 9 days.
I see tons of older planes on Trade A Plane and Barnstormers, for sale, for 6 months or more, not moving.
Markets vary, but the ones not moving are not worth the asking price IMHO. Do your own due diligence.
My suggestion is get a 2013 or newer, or discount the heck out of an older one that needs newer EFIS glass to be 2020 ADS B compliant, because what ever you save up front, you'll be spending on new EFIS glass and time to rip out the old and install the new.
YMMV, this is all IMHO. I'm retired, and don't enjoy working much on electrical or computer based stuff, or crawling around in tight spaces, being 6'5" tall, I can't bend or contort around, like I used to when I was younger.