While I agree with Clive and Theresa with everything they've said, I have to add that a good HOB protein skimmer is pretty much a necessity as well. I prefer the protein skimmer to an airline in fact, as it does a better job of oxygenating the water while skimming out the DOCs (and there will be a lot of them) a seahorse produces. A bare airline helps too in areas that might get low flow, or dead spots, but a protein skimmer that turns the water over a couple times an hour combined with a HOB or cannister filter and a single powerhead should be pretty effective.
A 38g bowfront is a good choice for a horse tank, wide for viewing and tall enough for the horses to dance in if they're courting. Theresa is right on the money as usual with horse choices: I have 2 H. reidis and a single female erectus in my tank, and both species are pretty good beginner horses.
So.....are you going to start a tank diary while you're building??
Forgot to ask -- what kind of powerhead do you have? I highly recommend Koralia-1 for a seahorse tank, but if you have something else, make sure there is a foam block covering the inlet screen, otherwise your horse might get stuck!