M50s are not that tough to bleed. No doubt you did it properly. If the car overheats your best bet would be to replace the thermostat - even if it is not the original problem. Make sure you drill a bleed hole at 12oclock on the new one. Makes bleeding a cinch.
I believe the fluid should travel from the upper to the lower rad hose. Therefore if the tstat and water pump are working properly the rad should cool it down towards the lower hose. From everything I read not sure I could rule out water pump/fan clutch/aux. fan/rad blockage.
You can run a head gasket test pretty easy. Go to autozone or other and borrow their fluid reservoir. By a bottle of HG fluid, stick it on the rad and it will change colors if gas is in the coolant. Great for peace of mind.
Also, you can jump the aux fan with a paper clip and leave it on high when you drive to keep the engine cool. (No it is not a fix).