In US mpg terms, my '92 would do 28 at a steady 75mph. That is pretty close to what the handbook specifies from new. It was a 5 speed auto and had slightly better mpg than the manual 5 speed. I would think a later (vanos) motor would be more economical though I suspect some advantage may be lost if it is coupled to a 4 speed auto.
Plastic water pump impellers and thermostat housings really need to be treated with a little caution. Otherwise, the cooling system is like any other - let it overheat and it may well repay you by blowing the head gasket.
None of this is ground breaking information. As mattyb says, you will find more information if you do a quick search.
And welcome from me too!
Oct '00 E46 330i. Feb '92 525i (departed)