I'm sure Doug can arrange a mount change for the back at a reasonable price;
Unfortunately probably not. Mount swaps are contingent on availability and the availability of P25+ is not great. It would also be $3k if available, which is unlikely to be a good net value. Someone with an IQ180 wanting to change mounts - that often makes sense, and with an IQ2 back you automatically get a one-time free mount swap. But not with a P25+. With that you really want to get the mount right upon purchase.
When searching for medium format, regardless of your budget, you have two paths:
A) Find a good deal, and then look to see if it's a good fit for you
B) Figure out what's a good fit for you, and then look for a good deal on it
Many people start out on (A). But in eight years working at a Phase One dealer it's clear to me that (B) ends with a happy buyer more often.
My advice is not to make your decision based on this specific Craigslist back. Instead figure out where you want to get, and then start looking for a good deal. It's a big world, and with some patience (assuming it's not urgent), you can find a good deal on anything you narrow in on. It's very possible that the right system for you is an H2 and a P25+, but it's also possible that it is not.
I would also selfishly and self-interestedly suggest that you consider additional factors into what is a good deal. For instance warranty, dealer support (for all the questions you have, and any training you need/want), condition of accessories, assurance the back is in perfect order (no IR scratches, no fungus, operational even for use-types the previous owner didn't need, firmware up to date, calibration of sensor good).