So far, I have not seen any marine tank with top box. Sea horse is very hard to keep so u sure u wanna try? for corals u need chiller. From the looks u are going to budget so maybe just fishes will be fine. Corals maybe mushroons only...
There are external skimmer which you may like to consider. FYI, seafood resturants also use skimmers for their live seafood tank. so wat do you think? do you need a skimmer? There is Weipro, jebo (i am using), etc etc... if budget is a constraint. But for me Jebo has proven to work very well. another reefer here say Weipro is proven for him.