Water in the oil

[Lynn Eades]

Tony, A corrosion inhibitor prevents the build up of rust inside the block head etc.(assuming it is clean to start with) this then allows an efficient transfer of heat to the water, and away to the air at the radiator. The inhibitors do require careful sealing as they are inclined to leak more easily than plain water.
Keep in mind that 90+ percent of engine failures stem from cooling system problems. Does your engine have steel core ( or frost,or expansion) plugs? Is it worth trying to prevent any further internal corrosion of your block or head etc. from what you say the tin plated retainer for your thermostat had rotted away, so the inside of your block must also be slowly corroding. All that stuff helps to block up your radiator(don't forget to flush it, from top and bottom)
Yes I would recommend an inhibitor. An anti freeze type usually should be replaced yearly, whereas something like Fuchs engine coolant(German brand) might outlast you. (you mix it 50% with your rain water)