I'd go big with a furnace to start, the Lee 20-pound being what I have. I started with a 10-pound, and have recently been given two more 10-pounders by people who have out-grown them.
Starting with a 2-cavity is not a bad way to start. For production I use a 6-cavity Lee aluminum mold. Aluminum molds tend to have a wear pattern known as "galling," which can be dealt with by a special lubricant available on the Cast Boolits board known as Bullplate lube.
Beware of Zinc wheelweights.
Material handling can be a problem, as can be Lead throughput. I empty my mold into a 5-gallon bucket full of water, which causes harder bullets and less dents from bullet collisions. When using a 6-cavity mold, especially .45, I use a 20-pound casting/melting and a 10-pound furnace melting furnace at the same time, or I suck the 20 dry quick.
CDD