I think a 70 or 100 qt blue cooler mash tun would be fine, and when you want to brew a bigger beer . . . second mash tun. You can probably get two 70 qt blue coleman extreme coolers for less than $100 (currently $43 each
). If most of your beers are less than OG 1.060 I think a 70 qt cooler would be fine for getting 24 gallons. The second one could then be used to make a higher strength beer in the 12 gallon pot. Or if you wanted 24 gallons of bigger beer, both mash tuns can be run off into one pot. And for really really big beers (over 1.120 probably, standard by Fred's measuring) then you can supplement with sugar or extract.
If you're good you can make really good beer with extract, but for what it's worth, in our recent competition the best extract brewer I know won three 3rd place ribbons.