what's the ideal battery type/size for a '47 model B?

If it is traditional 6 volt, group 1. I seem to remember some of them used 2 6 volt batteries to achieve 12 volt, dont hold me to that, my knowledge here is very limited.

There are 12 volt generators. Just be mindful that most generators were positive ground.

Starters dont care about polarity. Turn same direction no matter the polarity.

Onlything that does care is if you use a GM alternator. You will kill the diode tree with a positive ground. Alternators require negative ground, no rewiring necessary. Only thing you might consider is a balast resistor for your points.