James,
Edit - read the wiki first. There's a ton of stuff on changing the coolant that may well be at odds with what I've written.
Did mine a few weeks ago. I think that 4 years is as long as you'd want to wait to change the coolant. 3 - 4 years would be about right. It's a simple job to do, if a bit messy.
I prefer to remove the large hose at the bottom of the rad rather than attempt to undo the drain plug (to save anyone asking why, there's a post on the site somewhere from the last time I went to change my coolant by undoing the drain plug on the rad. It's been re-soldered now, and is in place where it will stay from now on... . Suggest you remove the hose and drain that way).
I always warm the car up a little to get the juices flowing and motivate any residue that would otherwise stick. Make sure you open the heater core to the coolant also to ensure you get full circulation. (But don't over heat the coolant; it gets hot quickly, and pressure will build up. Be careful opening the rad cap if you end up putting the tank on again first. Actually, I suppose you can't warm the coolant up with the tank off. I'd stick it on first, personally, then warm up, then drain).
When you remove the hose, 7-8 litres of coolant will spew out. You should flush it but sticking a hosepipe in the coolant tank and flushing water through. I have very hard water, so I used bottled water to do this. If the radiator is cruddy (yours shouldn't be as it's recently re-cored) you could reverse flush it (put water in the other way). I also elected to remove the thermostat (easy) and check its operating temp (put in a saucepan of cold water with a thermometer, and heat up. It should open at the temp stamped on the thermostat body. If it doesn't get a new one). (Obviously). (Since it can't be fixed). (They're cheap). I also flushed through the thermostat pipes to get the old coolant out of the engine water jackets.
Replace the hoses, tighten clips. Replace thermostat. I premix my coolant with distilled water (actually got from the tank on my dehumidifier) checking the specific gravity and getting it right for my temp range, then, having got the right temp range, pour it back in via the tank. It should take about 6 litres then it will be full. Put the expansion tank cap on, and run the car gently bring it up to temp. Check for leaks from the hoses you undid during this time. Stop the car, top off the tank. May need to do this more than once. The old coolant is an environmental hazard and should be disposed of correctly.
Check for leaks. I think that's about it.
I didn't use MB coolant, but only because of the cost here. I just used one that's recommended for MB with alu engines.
As for the tank, it doesn't get hot enough to require hi temp paint, if you mean manifold paint. I used normal hammerite on mine and it's been fine. Wouldn't have bothered with primer. It's copper I think. I'm sure someone who knows something about paints will tell me I'm wrong!
JH