Make it a brewpot. Stainless steel boil kettles can get pretty expensive, especially a 15.5 gallon kettle. Of course this is probably going to move you outside with a propane burner. I'd make your keg a keggle, that's what I did. A mash tun is cheap to make so don't use it for that, not at this point.
You'll probably want to get a sight glass and 1/2" full port ball valve. You'll have to cut a hole in the top of the keg and drill two holes, one for the sight glass and one for ball valve. If you don't have the tools, you can get an angle grinder from Harbor freight with and a pack of cutting and grinding disks for about $30 and a step drill bit kit to drill the holes for around $13.
If you're interested in that idea there are a lot of DIY instructionals on YouTube and HBT. Somebody here will probably direct you to those if you are interested. I would but just haven't figured out how to put YouTube videos on here, yet.