I have an SSK-4 and use it exclusively for C41. I agree Jerry is the man when it comes to Phototherm. He's not only an expert at diagnosis, I think he engineered the incremental improvements they made to things over the years (the 4x5 holders for example - I have 3, one is from a later revision they made, but he showed me how to update the older 2 to match, and why).
For spares to acquire: I'd think a spare drum, spindle and 4x5 holder, ideally. Maybe a few other things too? In general they are reliable machines, I've never had to mess around with mine as far as calibration and such. I have 3 sets of drums/spindles/4x5 holders so it's nice to be able to load up 3 sets and just run things back-to-back, speeds up processing a lot.
For the touching/marks on 4x5 - the latest version of the holder solves inconsistencies like that - I saw those a couple times on my older holders, but even there it was rare (< 5% of the time).
I doubt the Mod 54 would work, the film would likely not stay in position.
Agreed that it's more robust than any Jobo I have seen or used, and parts are less expensive and easier to come by (the standard ones like tubing/etc. - the custom parts are still not cheap). All in all the best film processor out there, I think. A shame they are no longer being made. :-/
Enjoy!
-Ed