I would suggest 15 gallons. That way you have more than enough room for any size boil of a 5 gal, and you can also do 10 gal batches if the need ever arises. The price difference between a 10 and 15 shouldn't be much.
I know you said you wanted SS, but for comparison sake, I got a 15 gal aluminum kettle for $70 from a restaurant supply store. If money is an issue, there is nothing at all wrong with aluminum.