USGI 30-rounders should be the backbone of your mag collection. And brand new, USGI-contractor 30-rd. mags just dropped in price from 30-50% for non-State-AWB people.
Get a few jungle clips ($5-8) or other mag connectors to bind 2 30-rounders together.
Also get a few 20-rounders so you get a feel for them. I like both the USGI 20s and the Thermold-Master Molder 20s.
Beta Cs are much more attractive now at $250 than $600, or even Ruben Mendiola's longtime price of $389, before he sold out his supply. Never tried one myself. Maybe you could hook up with an ARFCOM member in NV that has a Beta-C, to try it out.
You'd better offer to fill the mag, once for you and once for him.