Why not run a smaller separate LiIon battery for the fans, because they are the only other high load? That way you could keep the cost down and main battery for starting duty. Would also allow separate charging of either battery when needed and possibly running the fans with engine switched off.
I found out that a local shop sells an alternator similar to the Hitachi, Denso etc. But this has a pulley for a 13mm fan belt. Any suggestions on where to get a pulley that will fit the smaller fan belt?
Regarding using two batteries, I am only allowed to use one.