You could do either. FOWLR is gonna be easier and a bit cheaper. Fish tend to be hardier than alot of corals(although I guess that's debatable depending on the coral) so although you have to maintain water quality, you get a little bit more leeway. If you do a "mixed" reef(meaning softies, lps, and sps corals) you'll definitely have to maintain very good quality if you want things to thrive and look pretty.
And then there's the expenses. For a reef, lighting is very important. Prepare to spend anywhere from a couple hundred to well into the thousands depending on how big your system is going to be. If you go with a midsize tank, you'll probably spend a few hundred. Lighting doesn't matter for a FOWLR. A skimmer and an RODI unit are other items that you will want for a reef. They can be pretty pricey depending on which model you go with. You don't absolutely need these 2 items for a FOWLR, but I would still use em just for piece of mind. These are just big ticket items that I'm thinking off of the top of my head. There are tons of little pieces of equipment that aren't really that expensive individually(powerheads, test kits, plumbing parts, etc. etc. etc.) but the expense will add up.
Basically, you can start with a FOWLR if you like, alot of reefers do. Just keep in mind that when you want to start adding corals, it's going to mean buying more equipment. I have no idea how in wall aquariums are plumbed and this might make a FOWLR-to-Reef conversion harder than usual, but regular reef tanks usually have an overflow and a sump where the equipment is hidden. It sounds like you eventually want to end up with a reef tank so I would plan the design of the tank with that in mind.
! Then again, I suppose you can slack off a bit with maintenance with fish only.
