Sounds like some people have never seen a Weber...
Don't sand blast it. That and a wire brush are liable to ruin/scratch the finish. Aren't Webers porcelan coated? I'd be careful with that SOS pad, too.
Do a hot and fast burn to burn off whatever is in there. Then wash out the inside with a lot of soap and water, but don't do too much manual labor, more of a soapy rinse.
Get a large plastic bag/garbage bag and put the lid in it, along with a small container of oven cleaner. Put the container of oven cleaner in the bag but don't get any on the lid. Close up the bag and leave it at least over night, 24 hours would be better. Next day carefully open the bag outdoors with lots of air and remove the lid and rinse. Close up that bag and container of oven cleaner and throw it away. If you did it right, and the previous owner didn't scratch the inside coating, the lid should be clean, or almost. The fumes from the oven cleaner did all the work and you never had to play in the stuff.
Let us know!
BOB