I've used the high temp paint at the local home store and it usually takes 2 cans. I don't think you need primer if you give the drum a good wash and dry first. I think that paint is usually like a primer anyways. Another option is to have it powdercoated somewhere. This might cost you $50-$75, but it will look good and maybe last a little longer than the paint. Make sure to ask if they do high temp powder coating.
I have built 2 drums now and have not used primer. I use high temp paint from the auto parts store. Makes for easy touch ups in the future. I have not had any problems with the paint so far. Has been 2 years.