If you could find a food grade barrel I would buy it instead but if that barrel is all you can find then it can be used with a little bit of extra work. I personally wouldn't ever use one but I have food grate barrel available to me and for some people that isn't an option so you have to make due.
The only thing I would check for is to make sure it does not have an epoxy liner. So long as there is no liner, its no different then a fuel oil tank or a diesel tank. Slightly different then a propane tank but not much. Both of the comp rigs I cook on are old diesel fuel tanks.
There are many options to clean it, I used dawn dish soap, you know,,,, the one that cuts grease lol.