I personaly from experiance would recommend a sump set up. I started with HOBs they work not gonna say they dont I prefer a sump set up because the HOB
filters will cause spatter and that spatter causes salt creep on the tank. Sumps dont do that. Also with the sump system it elimonates alot of the stuff hanging in the tank its self. It provides filtration and a place for protein skimmer( a must for salt water) the sump can be set up in the cabinet under your tank so that everything is hidden. You have to filter along with the skimmer so that puts everything in one spot and not on the back of your tank.
Fish only requires more work because the live rock isnt there to help with biofiltration. Fish with live rock is easier to maintain filtration wise.
When you get into corals you should research lighting because they have special requirements that standard bulbs cant do. Lights for corals are expensive so be prepaired for that.
Initial set up for salt water is much more expensive than fresh water maintenance is also more expensive.
I use my tap water which has a natural high Ph. But you should test your tap water for nitrates and phosphates. If they are present I would recommend an RODI
unit to do your water or find a supplier you can purchase water from for water changes as salt water in my opinion is more sensitive to the phosphates expecialy.
Research the fish you like see what their maximum requirements are and use that to guide tank size and needs.
Also consider your wallet size lol because it can get pricey quick.