This always turned into a beer mortality issue, you see the $50.00 you put down on a keg does NOT cover the cost of the keg, in fact the keg is owned by that brewery, if you keep it and hack it up for a brew kettle it's destruction of property.
Now what do you do when a beer distributor will not take a keg back? I my case I had two Kegs that I tried like hell to give back but the beer distributor would not take them, I'm NOT mailing them back the Germany, so I kept them and turned them into kettles.
I guess what I'm saying is try like hell NOT to purchase a keg of beer and not return it when empty, put your self in the breweries spot, a new keg is like $150-200. Breweries will decommission old kegs, sometimes they can be purchased through a beer distributor, also one of the big home brew stores online has in the past sold used decommissioned kegs.
Craig's list is a good place to look, you can find some for a good deal, while they are probably not decommissioned it's still better than steeling one from a brewery.
In my Honest opinion there are much better kettles then kegs, kegs are not designed to heat liquids and as such the bottoms are ill suited for efficient heat distribution. A quality kettle will have a Flat triclad bottom, this type of bottom will heat more even and efficiently. Honestly if I could redo my brew rig I would have saved up for real kettles rather than cutting up kegs.
In the end do what you feel is right for you, I hope I have provided some helpful insight and not sounded like too much of a finger pointing weenie.