In my opinion the right way to do a seahorse oriented tank is too do a normal FOWLR
or reef setup for the first year or two, by then the tank will be seasoned and established enough to transition it over to a seahorse focused tank.
They need a well established set-up with pristine water conditions, and very docile tank mates, if any.
Dwarf seahorses are small enough for a nano tank and are not cost prohibitive, but only live about two years max.
Larger seahorses can be had, but be prepared to pay a handsome price for them, well over $100, but they grow to a pretty good size and have a longer life span.
Seahorses are just about one of the trickiest critters to master keeping, octopi are about the only thing I can think of more difficult that are regularly available for home aquaria, that and jellyfish.