I had an old 5 gal cooler, so that's what I started with. I soon realized it was way too small for most of the beers I make, and now use it as an HLT. Even with a really thick mash, a 5 gal cooler will only hold about 14# of grain. The question is do you want to be able to brew beers with a higher OG than about 1.070? Even my 10 gal cooler seems a little small for some of the bigger beers I've made.
I've found that even the SS hose clamps tend to corrode after a while. I use plastic zip ties.