Keep your eyes open for the 35mm lens. Fabulous! Approximate field of view of a 20mm on 35 film, and really minimal distortion.
I've used an Epson V700 scanner with the Better Scanning medium format carrier with anti-Newton's ring glass and gotten very good results, far better than the stock carriers. The initial set-up with the Better Scanning is tedious, but the difference is significant.
Do consider an extra film insert or two. The 220 inserts are typically cheaper than the 120 variety, and can be "converted" to 120 in a few minutes. See this link:
https://www.pentaxforums.com/forums/pentax-medium-format-645-6x7-645d/101062-...agazine-2.html
Re: Mirror lockup. Luminous Landscape reported on that using as I remember a 600mm Pentax 67 lens via the adaptor and found no evidence of mirror shake in the 645n as compared to the 645nii
As far as tripods, my Manfrotto 3001 has served well, although it's not very tall.
Don't forget that the 645n will handle the Pentax
TTL flashes as they were intended to be used. With the AF400T and the external power pack you'd be loaded for bear. I think perhaps one of the current Sigma flashes will do TTL as well as PTTL.
You will enjoy the 645, especially the queries "What is that and how many megapixels?" Better work up a good answer!