Give me a day or two to go through your list. Most parts you mentioned are readily available, new from your local parts store or via eBay.
MC/WC - if not too expensive have them re-lined if that is what they need. Otherwise go with a 20 gauge shot gun copper bore brush to clean them up. I would not use a hone, ruined a front w/c, as they are a little too much for the lining of the MC/WC.
New - BrakeBest offers a new Brake M/C with and without resevior or go with eBay (need to look for best price)
Clutch Slave Cylinder - sorry have not seen this one too often. The CSC from a 520 DOES NOT work! Been there, done that, gutted it, and threw it in the 520 parts box. I also do not know if any British CSC will work.
Water Pump - go to NAPA parts on line. Look up Datsun 520 with J13. This will give you a part # that can be crossed referenced by another store. New, not OE type, but will work.
Thermostat housing - go with a MGA/B Sprite/Midget, they will work, same bolt pattern. I just need to verify this one, but should work. eBay, Moss Motors or Victoria British.
Rims - any Datsun from a 223 (222 has 15 inch and I need one!) up to a 720 will work. All 6 bolt, 14 inch rim. If desperate, might look for a Datsun Trailer on your local craiglist.
Front W/C if you cannot source the OE 3/4 w/c might go with the 13/16 from a 520. They are still SAE with 3/8 inch x 24 threads.
I'm doing my best to be at Long Beach JCC in September with MNSTR 0.
Let me know if you need any parts verification.