your best bet for seahorses is to read up at seahorse.org. I can tell you that some people swear by skimmers others dont for seahorses. If you get a skimmer make absolutly sure you dont put bubbles into the tank. The best thing you can do is put a fuge/sump in for that tank where you can put the skimmer. for corals just make sure they arent stinging corals and dont put sponges in there. If she gets seahorses make sure she gets captive bred as wild caught usually dont live more than a couple years in captivity. If you are going to keep any corals you will need to upgrade the lighting eventually. you should be able to keep mushrooms under that type of lighting but thats about it. also with seahorses you have to watch the temp. there can not be a big swing in temp like you can have with most fish. some tanks will require a chiller for certain seahorse species. I cant think of anything else right off hand. as for sand at least 2" should be good which is like 40lbs i believe?