You don't have to buy a hole saw. I just use a kitchen knife and a pair of sheet metal snips. Trace the pot for the hole. Stab or slash with knife. Then cut out circle with tin snips. Easy.

As far as size of pot, it depends on big you plan on growing. Latewood is right to use a large pot if you're growing large plants. You will need the weight, otherwise the plants will tip over. You could tie them up. Weight wil definitely be a factor.