Think most people missing the point, there is no harm in buying a 30gal kettle, and then using it for smaller batches. The Original poster was concerned with having to buy gear in the future, and the big kettle doesnt hurt small batches
I do 20 gallon batches. Started at 5, went to 10, now at 20. I have more beer than I could ever feasibly drink. Especially since my wife divorced me, and my GF has celiac.