Alan,
Definitely go fot the KF! The 24v electrical system is really not needs on this aircraft unless you just want to spend a lot of $$$..
Batteries cost more, anything electric will cost more. Does Rotax even make an engine with a 24v system? I don't know that's why I ask.
Most EFIS systems today will run on 12 or 28v but the wire size won't change.
VPX is overkill.
Wiring is not complicated but can be intimidating. The one thing you will need is the proper tooling, I.e. wire strippers and crimpers. Do NOT use hardware store strippers and crimpers! These are OK for house or car wiring but they do damage the wire.
Regards
Jeff