Agreed, make sure they actually own it. If you're getting it from a brewery I wouldn't plan on putting beer in it - if it was still good for that, they probably wouldn't be selling it. I sometimes pick up kegs from people who obviously stole them, then return them. Some places are reluctant to give you money back for a deposit if you don't have a receipt, but I know a guy who'll take them and return them to the distributor. They pay him, he pays in me in beer (he owns a bottle/keg shop). Win-win.
If it's legit, you can definitely turn it into a boil kettle or HLT. Or make a grill or smoker out of it.