Horsepower consumption of the factory fan is minimal when not engaged. You will save some, but I suspect you will not notice it.
The new fan will be quieter than the engine driven on, but you will still hear the air moving, no way to make that any quieter. You will have to be the judge of that.
The fan won't clean the engine compartment any more than driving at 50 mph.
However, there is no real problem. Most newer cars (and cars with transverse engines) don't have engine driven fans anymore. You won't go wrong UNLESS the fan stays on all the time. In that case, you are using more horsepower than the engine driven one.
That all said, there is no real reason not to install one if you want. Just don't screw around with the thermostat system, it works just fine as is, all you are doing is driving air across the rad at slow speeds with an electric fan rather than an engine driven one.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!