Welcome to the boards
A 75G would be a good starter tank. Although if you know you are going to be in the hobby for a long time then I would say it is best to get as large as you can afford and have space for.
The best equipment you can get will vary upon what you want to do with the tank. For example, if you want to have fish only then you wont need much accept for maybe a good protein skimmer. If you want to have a reef tank and keep some corals then you start needing better equipment. Lights, etc. If you want to have anenomes, clams, or stony corals then you need the stronger MH lighting.
Here is a general list of the equipment
Tank
Lights - PC, VHO, or MH
Powerheads
Heater
Filtration
Protein skimmer
Sump / refugium, or sump fuge combination
I have heard there is a great book for beginners called, "The consientious marine aquarist" which would be worth a read.
Be sure to ask lots of questions.