Welcome to the forum!
SS is expensive, has poor heat distribution (hot spots), but lasts forever, and looks cool.
Aluminum is cheaper, has better heat transfer, but doesn't last as long, is softer, and doesn't like harsh chemicals.
I have a 15 Gal SS keg with the top cut out that I boil in and I love it... I couldn't tear it up if I wanted too. Cost me $20 on Craig's List, (I had to cut the top out, but that wasn't a big deal). It may take you a few days, or a week or two, but I see kegs on CL all the time.
Other than that, you're going to have to drop a few bucks to get anything new that is going to be worth having.
Do some searches here on SS vs Aluminum, and cheap boil kettles...
Alright then...