Goose, you'll hear a million opinions on this matter. Here's mine:
If you're a novice, buy a new set. They're not that much more expensive than a used set.
If you're on a budget, you simply cannot beat the Lee Deluxe Die kit for the value. For the price of other brands' two-die sets, it will give you three dies plus a shellholder in one package. The third die is the collet neck-sizing die, and it is worth about $25 by itself -- and, in reality, it is probably worth its weight in gold. If you're loading for a bolt rifle, you can use that neck-sizing die and don't even have to lube the cases. I use if for my target loads.
However, you only want to use a neck-size die for brass that's already been fired in your rifle, for rounds that are going to be used on the range, and for rounds in a bolt action rifle. In all other cases, you should use the full-length size die. It's just great that the Lee set gives you that extra option.
Otherwise, any new set of dies by a reputable manufacturer will work with your RCBS press.