Bigger is better and you will eventually develop two foot-itis. I can fish four in an 18ft, but sometimes I wish I had ordered bigger.
Aluminum will do fine in the salt as long as it gets a reasonable amount of care. That would be a good rinsing for both boat and motor. If you can run it in fresh water afterwards, so much the better.
When shopping, look for a boat that has the keel completely welded stem to stern and on both sides. G3 welds their hulls ona center seam and then stitch weld a cover plate to cover the center seam. This collects saltwater and if not rinsed, will give you grief. Also, if you are going to be in salt water, get a galvanized or aluminum trailer. A painted one will be eaten up in a couple of seasons.
I take my Seaark to the coast and when I get back to the lake everything gets a dunking. Corrosion X is also your friend.
2015 Seaark 1872 MV CC Etec-90
Two Helix 12 CHIRP SI's, , MinnKota Riptide ST80/i-pilot Link, Bob's Hydraulic
Bitter Gun Owner