There may be some with an access door, but almost all that I see don't. You do need to remove the food to tend the fire, but the nice thing about a UDS is that you rarely need to tend the fire. I always fill my fire basket with lump charcoal and a few chunks of wood, put a half of a chimney full of lit charcoal on top, and it will stay lit without any tending for quite some time. I've done many 10-12 hour cooks without having to tend the fire. One thing I have noticed on my UDS is that it likes to stay around 235-245. When I try to keep it at a lower temp, like 225, I tend to choke out the coals and I have to do a lot of fire tending. So I've embraced 235-245 for my UDS. Also, if it is windy, I sometimes need to tend the fire.
As far as how much food. I've done 2 briskets at once; 4 pork shoulders at once, and I think there was still room for 2 more; 6 chickens at once; 8 slabs of ribs at once, in rib racks; and many combinations of the above. If you use a Weber lid rather than a regular drum lid you can do turkey and beer can chicken. Also with a weber lid, you can add another grate and have two levels. I added a second grate, but it was a pain to pull the top grate off to get to the food below, so I stopped using it. If I need to cook that much I'd rather have two smokers going.
The Pickled Pig website has a good outline of how to build a UDS. He also outlines the cost, which I've found to be pretty accurate. I've heard of people saying that it only cost them $60, but they must have had free drums and scrap steel and hardware, which I don't. It isn't too hard to build. In my opinion, burning the drum out and removing the paint is the worst part. It can be time consuming and the soot gets everywhere. Everything else is pretty easy.
