Take it to your local motor mac. They will throw stuff in the radiator, then ask you to drive a bit and a come back. Then they connect it up to a machine that flushes the crap out. Endlessly more effective than buying Wyn's radiator flush.
Remember to ALWAYS use antifreeze, if you did you would not have issues with radiator or the gasket for that matter. If you are getting new valves there is another cost (included in my estimate) of haveing the rockers adjusted, apparently its quite a difficult job,and mine was done at the same engineering co that fixed the corrosion damage to my head (caused by not useing antifreeze).
While you are at it, inspect all rubber hoses for the water system for brittleness and cracks, also inspect the metal pipe just above the oil filter, it is the pipe comeing out of the waterpump, I recently had to replace mine as it started to leak due to corrosion at the joint, it was only like R100 or so.
Also check fuel lines and the pipe between the carb and vacuum advance, buy 2m of 5mm fuel hose from Midas and you can re-do all those little pipes in the car. My vacuum advance pipe was realy shot - as was the advance.
Other things that will go soon if they have not started are the clutch master and slave as well as the brake master as well as the little piston jobbies on the rear wheels that actuate the brakes (R60 each, and kits for the others much less), basicaly everything rubber on these cars starts to degenerate at 15 years or so. Even my suspension bushes are now getting tired. You can realy do the clutch jobs yourself with very little experience (like me), just make sure the rubbers in the replacement kit looks EXACTLY like the old ones, had some issues with cheap fakes.