Unimog Club UK

M130 oil pump will be difficult maybe a Merc part for that one as its not Unimog part but you could try Manfred Prince at Westfield 4x4 defiantly for the 404 parts you require.
If you find the master cylinders expensive you can have them bored out and stainless sleeves fitted there are a few companies on the inter web offering this service.
I have always changed my brake fluid religiously to help prevent corrosion but if your going to the trouble of a complete brake refurb and the Unimog could spend a long time standing check out DOT 5 again Google it's used in railroad Unimogs that are used infrequently.
Moose