filter and skimmer is fine for a FO
system. Any skimmer is better than no skimmer at all when considering water quality (although it might not make sense for your wallet). Larger tanks do not do well with HOB
skimmer or HOB
refugium. Bigger is not always when you cannot afford these kinds of things.
If you want to do an easy FOWLR
with refugium, then you can try setting up a small tank. Perhaps a 40g tank with live rocks, HOB
skimmer, and HOB
5g refugium. No plumbing required.
If you want an easy large FOWLR
with refugium system, then you can buy a big tank (75g like you were saying). Then get a HOB
overflow (CPR is good) and a prebuilt refugium (such as a eshopps model).
You see people building and plumbing their sumps/refugium together because its saves LOTS of money and everything is built specifically for their system.
Also I started out learning about sumps/refugium and plumbing after reading/answering people's question on this forum (I had to look up the answer on google in order lol). And I also learned when analyzing how the manufacterer's equipment work and reading the reviews. Google is your friend. You'll find most answers faster on google if you think of a good specific question for the search engine.