You don't bleed it by opening the 'drain pipe'(?). You remove the expansion tank cap and run it up to temp from cold at idle speed. Once the thermostat has opened you can refit the cap.
Thermostats can stick. it does sound like an air lock or partly blocked rad ?
Is the expansion tank cap okay? Is there a seal inside it? A faulty cap could cause the problem too.
If everything checks out okay it may be the pump is partly damaged and just not circulating the coolant fast enough - impeller loose on shaft, or broken blades on the impeller.
Sounds like the wires between the body and door has broken. Cut the rubber shielding and have a good look at the wires. Quite common thing to go. If you open the door and open window squeeze or wiggle the wires between door and body and you may find window will work when you move the wires. I doubt very much that it is the motor or switches .
metal can wear down even against a rubber seal especially if a little grit or metal deposits get in between the seal and shaft as already been said by stef.
the seal should be a nice tight fit on the shaft. when you slide the seal on the shaft it should be tight then as it get to where it sits does it get easier to get in place ie the last couple of mil is looser than when first putting it onto the shaft if thats the case its the shaft that wore down i recon.
remove plugs and fit new. they will come out even if cold. if they dont make any diffrence replace the ht leads as they fail. as it was run to empty i would change the fuel filter as it most prob needs doing. its located under the car near the rear nearsid wheel. it looks like a soft drinks can
if you have something in mind to change it to then go for it. good price for the age i recon but dont change for the sake of change but as you said it's starting to get the odd rattle and it only gets worse.
sounds like a split hose. one the engine is warm put a pair of gloves on and squeez all the coolant hoses to find the ones thats failed and replace or cut down as the leak maybe on the clamping point and also check the expansion bottle and the bottle cap.
well done m8 the red ka was most prob a overpriced duffer. glad you got a better example and yes in the summer give it a real good going over with rust proofer especially around filler cap & seals and the bottom of the doors go aswell. put up some pics when you get it m8 .....happy motoring
before you make an offer take someone with you with some knowledge of mechanics and take it for a proper test drive over speed bumps . dual carrage way etc. it does sound like you intend on buying it so get it checked over properly.