The "over the tank" filters are usually called hang on back or HOB filters and they are decent for mechanical filtration and to move some water. 99.9% will tell you not to use an undergravel filter because they trap waste and cause nitrates. Bio-wheels also trap waste and cause nitrates and most don't use them.
Sand substrate is preferred by most. I'm not sure what dolimite is actually, but crushed coral is big enough that it traps waste and again can be a source of nitrates. It can be cleaned when doing water changes by siphoning and stirring it up, but it's not really worth the hassle. Sand is MUCH easier IMO.
You may want to reconsider the tap water and look into an RO/DI filter. Just do a search on these boards and you can learn all about them. Basically though, you have no control over what might come through the tap. And the water company is only concerned about making it drinkable to humans, not crystal clear for sea life (which has much higher demands). Also, tap water often changes in quality from season to season as the water company has to add or remove chemicals to adjust for changes in environement.
Wet/dry can also be a source of nitrates. I wouldn't use it.
Best mechanical filtration you can have is a protein skimmer. I suggest you don't skimp on it.
Don't underestimate how important biological filtration is either. That comes from your live rock and/or live sand. IMO, this is the most important filter you can have.
A tank is drilled so that water can overflow into a sump or refugium. In the sump/fuge, the water is then pumped back into the main tank. The purpose of a sump/fuge is to increase total water volume (chemical dilution), provide a place to store things like a skimmer, heater, and other filters, and in the case of a fuge provide a place for some extra biological filtration in the form of live sand, live rock, and even macro algaes which consume nitrates as food.
In summary, the important things here are....
1. Biological filtration is a must
2. Skimmers make life easier
3. Don't buy things that trap waste and increase nitrates. Some of them work when mainained well, but money is better spent on more LR or a better skimmer. Or maybe even a fuge.
