A 28G is a decent starting point, keep in mind, the smaller the tank the quicker bad things can happen if you don't stay on top of your PWCs. Personally, I understand your idea of plug and play, but I like to have the option to find better deals piece by piece and mot be stuck having to pay the higher prices for proprietary equipment. JBJ is a great brand and you should do fine with it.
A few pieces of equipment you will need:
refractometer (much more accurate than a hydrometer)
API makes a decent starter test kit (saltwater master's kit).
Don't use the test strips.
Take it slow!
Check out our articles. I prefer the more humane way of a fishless cycle.
Our advice is free, your LFS
generally is in it to make money....
Did I already say take it slow?