BMW: Leaking, Overheating, Smoking, Spewing, coolant tank, radiator replacement

Question
Ok, by the way this is an awesome thing you guys are doing to cut costs on the issues that come about to our BMW's. I have a 535i 89'and about three months ago while letting it warm up on a cold day came outside to find it spraying coolant. Had to get home so poured lots of water in the coolant tank. Made it home no probs, the next three months no probs. Started a few weeks ago to overheat while idiling, I would fill it with 50/50 and again fine. Nowadays it runs instantly to hot (after 20mins or so) and starts to smoke and leak. By idiling it at home, the leak appears to start on top of the Radiatior on the left side through no apparent opening (through a clip like part on top)then proceeds to do its dramatics which is other leaks and smoking. I obviously dont want to drive it now but want to fix this problem right the first time. What troubleshooting do you suggest to getting my car to run at normal temperature again. I was thinking thermostat and radiator replacement, is there any thing that I am missing and what bigger problems should I be aware of and how do I troubleshoot those.
Seems simple enough, leaking out radiator must be the radiator but after reading some of your answers whould just like to be sure. Thanks again for offering this helpfull site.
Adam

Answer
Adam,
Thanks for the kudos on the site. As for your car sorry we gave you such a crappy radiator. Unfortunately it is the best one I have seen. I replace lots of aftermarket radiators that are really new. Next to the radiator cap there is a philips head screw that is an air purge. There is also one on the thermostat housing that is metal, I think 8mm head. When filling the coolant open these screws and let the air out. Just let them burp until coolant comes out with no air. Then tighten them back down and bleed the radiator with the engine running. Any time the car overheats the thermostat is wasted, the wax pellet is damaged, so replace it. Make sure you put it in correctly because it can go in backwards in that car. The radiator is pretty straight forward to replace unless the car is an automatic trans. Then it is a bit harder. Save your time and take the transmission cooler lines off at the bottom of the radiator and transfer it with both radiators out. Replace the o-rings on the cooler lines and refill the trans. If you need help with that part let me know. The only other thing I would have you check it the auxiliary water pump. It is under the mess of coolant hoses and water valves next to the driver hood hinge. If you take off the 3 nuts on top the whole mess slides toward the engine and you can kind of flip it over. The black Coke can looking thing with a wire and 2 coolant hoses coming off it is the electric water pump. There is a square hole on the bottom that blows out if the pump freezes then it leaks coolant badly.  Sounds like you are on the right track with the radiator and thermostat. Also if you do replace the radiator put a new cap on it because the old one won't seal. And please never ever use a radiator stop leak in a M20 or M30 engine! It does terrible things.