Firstly it will depend on the type of saltwater tank you are thinking of.
Fish only
Invertibrates only
Mixed fish and inverts.
The tank needs to be the largest you can afford and have space for but a minimum should really be 30 gallons capacity.
Filtration is always a bone of contention but being old school I would go with powered undergravel and auxilliary outside power filtration.
There are a good selection of "hardy" marine species and if the tank is just decorated with sand, petrified wood and rockwork should the need for copper medication be required then it can be used without a problem. (Although after it's use you will not be able to introduce invertabrates, anemones or corals as the lingering copper residue will kill them).
