The water pump on the early E36's prior to 1997 were made with a plastic impeller which was prone to failure. In the 1998 and later models it was replaced with a metal impeller. I believe I have had to replace mine though. A while back my car overheated and the upper hose came off spilling coolant everywhere. After attaching the hose and adding coolant I had to re bleed the system to get the air out. I ended up replacing the water pump, thermostat and fan clutch.
You already replaced your thermostat so the other possibilities may be your water pump or fan clutch. The fan should spin freely when the engine is cold and off, and difficult to spin when the engine is hot and off. If it is hard to spin when cold and easy to stop when running then you need a new fan clutch.
Whenever you have work done on the cooling system such as replacing a hose, always , always re bleed the cooling system. Air in the system is the number one cause of overheating.