Is it a first SW tank?
If it's a first tank, I'd be worried about the size. I'd think you'd see every little fluctuation in parameters, but in only 5 gallons I think those swings would be that much more dramatic. I've never tried to keep a tank that small, so I am just guessing. If it were me, I would wait for PetCo's next $1 per gallon sale and get a 40b...I think that would make a great starter for the same price range you are talking about.