I have to agree, buy as big as you can afford.
Most people don't realize your number of fish is especially limited in a SW tank due to the low oxygen content of the water. That coupled with the sensitivity of the livestock can make what looks like a thriving active tank foul in no time flat.
Rule of thumb for a 50g is 10" of fish (nose to base of tail).
That's not much, and honestly can mean just one specimin in many cases.
It's all variable of course, some fish eat very little and produce little waste, other fish eat alot or are messy eaters...
A sump/predrilled tank add a good bit of coin to the mix but are aesthetically far more pleasing to the eye and provide some additional benefits in the way of filtration (more nutrient export means a more balanced tank).
Lighting is an important factor, but for the average person a 200$ VHO or CF can handle most any reef thing you'd want to keep (within reason..always check on the reqs for livestock). A fish only setup won't need any special lights, normal flourescent works fine (throw an actinic bulb in for the pleasing color).
Must get a skimmer...have to have a skimmer...don't skrimp here.. a good skimmer will keep things going well and stable.
Adding live rock and a decent sand bed to the mix will greatly add to the biological filtration of the tank, making breaking down waste products a more efficient process.
Is best (and least problematic) to go with filtered water.. a decent setup for that includes a ro/di unit (75$ or so), a powerhead, a heater and a 30g+ tub to store water in for water changes.
In my most recent large tank, a 90g bowfront, I got out of checkout isle for it and all the equipment for about 2300$... That included tank, stand, canopy, sump, return pump, skimmer, two powerheads with hydors (rotating tips for powerheads), two heaters, CF 265watt lights and all the tubes and powerstrips I needed.
Then add 40$ for salt (already had the ro/di unit), 10 bucks for sand (use the proper play sand from home depot), and 500$ for live rock (I get mine delivered straight from the ocean now).
So 2850$ just to start a new 90g with the potential for 85% of marine life out there (give or take, rough estimate).
It adds up quick... but I'll say it ends up cheaper then therapy! (I work in IT hehehe)
Oh, and I advise NOT to go with a 90g bowfront.. you need a blasted step stool to reach the sandbed, and I'm 6'1" with long arms (it's a TALL tank).
Sorry for the ramble..hope some of it helps somehow
gl and welcome to the salt!