I'm really no one to ask about sumps/refugiums (mines going to be made out of a hob filter, nano-reef look for the modded ac70's if that interests you). But you have established tank water, if you got a filter with some purigen or chemi pure in there, small light fixture, koralia nano or one, and another heater and some LR you've got yourself a 75 gal fowlr and a 10 gal coral grower

Flash has a good idea as well, that could be useful down the road if anything unexpected happens with some of your fish.