You're getting into major bbq theory. There is a book, the great american bbq handbook, it has plans. The site barbecuen.net will answer all your questions. I was planning a barrel burner like you, I have a welder and plasma cutter access. I had a plan together and then went to walmart to look for detail ideas. A bbq was already made that was nearly identical in function to what I was going to build and very cheap. The unit is stout and I have made some good pork butt, chuck, and chicken in it. I have a grill for grilling, this is a bbq.
Look at replacement weber grill parts for the actual grill surface. One of them will fit inside a barrel. You need an air inlet at or below the fire grate and a means for ashes to fall off the fire. All of your cuts need to seal tight like a woodstove so that you can control the heat. An exhaust should be large enough and there is no reason to ever choke the exhaust. You will need a thermometer to monitor the heat produced at meat level. You need a means to add/remove meat and to add/remove fuel without disturbing the other, this usually means a fuel door and a fire door.
Really, look at commercialy available bbqs. You will be surprised at the quality and if nothing else you can steal their ideas.