BBZ, KP is right, that 100/20 is plenty for you. Jon R is right too... future proofing is good. You are right too, picking what you need for right now.
As for the correct order of things... I'd disconnect the solar panels first, then the positive wires from the load and the battery, and then the grounds, battery ground last. Reconnect in the same order unless the owners manual of the Victron says otherwise (which it does, it says to connect the loads first - page 12)
MPPT solar charger manual (victronenergy.com)
Vic
Thanks Vic!