There's a simple calculation when decided what rated cable and fuse to use on any electrical installation, Watt divided by Voltage, therefore if your fan was say 120w, divided by 12v gives 10amp (in reality you'd probably want a 15amp), if it were on a 24v installation then it would be 5amp, you get the idea.
There are definitely 2 camps on whether electric fans are a good modification or not. I went to electric, but have now gone back to mechanical. There's an interesting article on the Canley's website:
http://www.canleyclassics.com/technical-archive/overheating
Personal choice at the end of the day.