I don't think you can go wrong with a 12 plus, a P1 100mp back, and lens.
The 12 plus is a beautiful piece of kit.
For the back, I'd probably look at the IQ3 100 to start. Compared to the IQ1 100, it has electronic shutter, which is really nice in the days of copal 0 discontinuance. Compared to the IQ4 100 it's a lot cheaper. The trichromatics supposedly have better color vs. the standard IQ3 100 but usually cost more $$. The IQ4 150 is the best way to go IMHO if you have the money because, being a BSI sensor vs. the FSI sensor of the 100mp-series, it doesn't show color cast issues as much as the FSI 100mp sensor with older lenses (like the discontinued schneiders). I've also read that the schneiders exhibit less distortion than the retrofocal Rodenstocks, like the 32 Alpagon, but don't have any hands on experience.
As far as lenses go, I'd look at the 32 Alpagon over a 28 SK/Rodie or 35 SK (or 35 Rodie) because it will give you the most movements while minimizing color cast issues on a 100mp sensor back (if you go 100mp).
I also am not sure I understand the allure of an XT kit (unless one has the money and wants to buy new and wants movement metadata baked into the files) and would go for a used Alpa/Cambo kit over XT kit just to save money, unless for some reason you need the X-shutter capability (which keep in mind requires the added expense of an IQ4 back over IQ3 for compatibility). You also lose tilt and have limited movements with the XT.
Have you reached out to our forum sponsor Capture Integration yet? They are P1 XT/Cambo/Arca/Alpa dealers and I'm sure could do a remote demo and show you around the various systems. I also think they may even have a used 32mm Alpagon in stock (albeit a LB version that would have to be remounted to allow for tilt use)....just sayin (btw I have no affiliation with them other than being a customer).