1) Fan cooling also increases cooling only by reducing the bottom oil temperature going back in to tank. Fan cooling is recommended from cost angle. There are users who ask for 100 MVA units only with ONAN cooling.
2)Yes.ONAF is to be preferred over ODAF (oil pump &fans).My factory regularly supply 500MVA 400 kV 3 phase autos &1500 MVA 765 kV Auto Banks with only ONAN/ONAF cooling.
3) Special low noise transformer cooling fans are made by almost all reputed makers. If you mention the area where you are looking, I can suggest makes. But better leave these to transformer makers who can do professional job
4) DM 61850,sorry to say, your idea is not safe. Any wet, old transformer, if overloaded can result in failure by bubble formation from winding.