Dirk, I'd pay a maximum of $15 for 3 plate holders and $30 for a Rada or Rollex roll film back, you'd also need to find some film inserts.
I'll check out what plate holders I have, there's two boxes and over 150 of various sizes & edge fittings, I've only done a quick sort and still need to identify them. I should have some for a Maximar.
With roll film backs you need to be careful as they were available for 620 film as well as 120. I really need to compile a good table of the different types/edge fittings with photos/illustrations & measurements. It'll take a few days then I can do more accurate identifications.