The smartest choice would be as big a tank as you can fit. The 4' 55 gallon (208 liter if thats standard there) tank is very popular. You can use a thin layer of sand on the bottom and use a wet / dry filter or a sump system, which will go under the tank, in the stand. That will provide the location for your protein skimmer, and it will also handle your 3 basic types of filtration - chemical (a place to put a bag of activated carbon), biological (bio balls that you never wash), and mechanical (a sponge to trap particles, combined with a filter pad to trap the really tiny particles).
Basically, its very similar to freshwater, but you will want to add a protein skimmer - a tube that you inject air into, and collect the foam into a cup. The skimmer is not popular with freshwater because it does not work without the salt in the water. But the skimmer removes most of the waste before it gets broken down, so it really helps and actually gives the saltwater tank a bit of an advantage vs a freshwater system.
Anything smaller than the 4' tank, you can go with an undergravel filter with larger substrate (crushed coral instead of sand, so the water will flow through it), and a hang on back filter that has a built in skimmer, or a hang on back filter and a hang on back protein skimmer as well. To go with an undergravel on a tank larger than 55 gallons would not be too economical because the cost of the filter plates and the pumps to power it, plus the hang on back mechanical/chemical filter will put you right around the cost of a wet/dry. Although the sound of the U/G system running may be a little quieter than the W/D...
The 4' gives you enough room to hold all your equipment underneath the stand as well, the sump will fit nicely and with all your hoses will have room to work around everything.
If you go with a wet / dry filter, you will need a way to get the water down to the filter, so there's two ways. One is a box that hangs on the back, called the overflow box. That uses a system thats gravity fed and siphons the water over the back and down to the filter. The other way is to drill the bottom of the tank and use a standpipe where the water will flow straight down. The advantage with the standpipe is that you can place the tank closer to the wall as nothing needs to be behind the tank, so if space is very tight, think about this method. Your dealer will need to drill the glass, make sure the tank is drillable and that you use a proper sized bulkhead and drill bit... this may involve a bit more research on your behalf. Overflow box is simpler once you learn how to start it, you will need to put water in the outside box and in the inside box, place airline tubing at the curve of the U in the u-tube, and suck the air out to get it flowing. It should remain primed, no worries there. This might sound Chinese but believe me if you do get the system and you were not shown how to start it these few words, should you need to reference them later, may be a big help. No need to memorize them now . You will need a good pump, perhaps a submersible pump with enough head to get the water back up to the tank, while not giving off too much heat into the water. Your other option is drill your sump and place an external pump beside it, advantage here is less heat transfer, but it may add a few decibels more sound to the equation.
Good lighting will really make the colors pop, so get a good bulb. You won't need high intensity lighting if your not going to have corals or other similar inverts.
You will need the high range ph kit, your freshwater kit wont get into the range you need. You will also need a specific gravity meter to measure the salt content, that's really simple to use, you fill it with water and a dial will float and show you how much salt is in the water, piece of cake. A bag of synthetic seawater salt mix, dechlorinator, some ph buffer, and off you go.

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