If you want to get rid of the corrosion and gunk inside, search the forums for "citric acid flush." It's been described in detail here. It's simple, inexpensive, and can be done at home with a few basic hand tools, a garden hose, and a bucket. Take your thermostat valve off and flush the engine circuit and heater, and flush your radiator. That's the whole cooling system.
Whatever hose, flange, gasket or fitting leaks or fails due to corrosion, fix and replace as necessary.
Find out how much coolant you need, buy concentrate, and prepare with distilled or demineralized water - you can use battery water, buy distilled water prepared, or distill it yourself. More coolant, lower freezing point, so you only need higher concentrations if your local climate is that cold. I use 33% in Karachi, 50% is fine also. The specifications for the coolant you use will include the range or a table of recommended concentrations depending on your climate.
Search the forums.
When I did this to two cars that ran most of their lives on water, all manner of rubber hoses, metal hose barbs, flanges, clamps, radiators, as well as radiator caps needed to be replaced. The corrosion was wretched and a pain to deal with on both cars, destroying basically every single component. Now that everything's done, they're running on different spec coolants for a good couple of years with zero issues.
