I have collected cast iron for decades and now buy very little.
It use to be much easier to get good deals on cast iron before Ebay. Now many thrift stores have their own Ebay account.
I was in a thrift store lately that did not have a Ebay account and watched people comb through stuff to sell on Ebay.
I new buy mostly what I need unless I get the rare good deal on something I could actually use.
I find it know better to buy some of it new and modify if needed with polishing.
I would recommend that you think about how you cook and how much space you have and make a plan for obtaining cast iron.
I picked up one dutch oven that sat for 20 year before I used it because I had others I always used.
Depending on your cooking style you only really need certain pieces in certain size ranges.
Like I now stick to six quart dutch ovens because of weight.
Cast iron, metal woks both India and China styles, CHAMBA COMAL, etc. there are many great ways to cooks and cooking tools to match.