Some background first, I want to have this UDS for a long ass time, and I want it to effectively (or semi effectively) be both a smoker, grill, and pizza oven. I get a kick out of multi use items. That said, I don't want to force it, and instead want to design it properly so that I can get better at all this stuff on one device. And I know... Pizza Oven. Well, I love pizza so hell yea!
I'll toss out few thoughts, and try to give some insights to your questions. A 'drum', UDS, BDS, can, or whatever you call them can be extremely simple or have more gadgets than Carter has pills. I think you could get away with using a drum as a smoker and a grill..... but a pizza oven might be reaching unless you blind bake the crust, but that is a whole different conversation. If you have not visited some of the popular drum sites, do so and take notes of what you like and what you don't like. Those would be
Pit Barrel Cookers, Gateway, Hunsaker (also sells parts), UDS Parts and Big Poppa Smokers (parts, kits, drums and has an
excellent 'how to' video).
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I think it will be easier to insert my comments within your list of ideas:
My design idea is as follows, with my rational for each decision after that:
55g drum: more volume will require more heat leading to smoke woods burning cleaner.
Yes, I believe bigger is better, my drums were specially made and are 43" tall and have a straight section without the expansion rings. I have supersized the charcoal baskets to increase my burn time. Mine double as a cold smoker, so I want plenty of volume.
Two racks: the first located 9 inches down from the top of the lid, the second 5 inches below that -
My drums came with two racks, I stopped using the lower rack after 6 or 8 cooks. Yes, a second rack improves capacity but tending the food on the second rack is cumbersome. You will need a folding table to hold the upper rack, and if you want to rotate racks you need to work fast and a second person really helps. Plus the 'original' concept of a drum is to simulate the 'open pit' style of barbecue so I do all my cooking on the highest rack.
Diffuser: ill cook up a cheap on and throw it 10 inches above the coals -
A diffuser goes against the 'open pit' style and restricts the little flavor bombs of fat dripping into the coals, which are what gives your barbecue a unique flavor. I tried one and did not like it at all. Hunsaker uses a 'vortex' diffuser and many owners really like it. They sell it as an accessory on their website.
Intake: I want to be able to get this MF hot, so I was thinking that slide Intake from UDS parts. I know for smoking its over kill, but for pizza we gotta be talking 600-800. That sound possible? Could I get this hot with two 1.5" intakes on the 55g drum?
A slider style intake (UDS Parts and Big Poppa) will get the temp rocking for sure. The bolt-on riser tubes work great and you don't have to bend down when adjusting. Many guys use ball valves. My drums have (3) 7/8" holes spaced at 120° to each other. I use electrical box knock-out covers or magnets to control intake.
Exhaust: I know that air in = even more air out, so I was thinking two, two inch square exhausts -
That would be workable, both UDS Parts and Big Poppa sell exhausts. Unless you use a hinge system I could see the exhaust fixtures making the lid unbalanced. My exhaust vents are (8) 1/2" holes in a circular pattern on the lid, also adjusted by electrical box plugs or magnets. My exhaust holes are generally open and I control heat with the intake vent holes.
Exhaust placement: this is where I am having the most trouble, again, cause I have no experience. Should the exhausts go on the drum lid? Center or sides? Should the go on the side of the drum opposite each other? Would side mounted vents cause hot spots where they meet the drum side? I must say I am leaning towards mounting the exhausts on the side about 1-2" down so that there is more radiant heat coming down on my pizzas from the lid. Do you guys think putting the exhausts on the lid would result in more or less heat coming down? -
This is usually a lively topic for discussion because when intake and exhaust work in harmony, you achieve a whirling, swirling kind of convection going on in the drum. This is why it's possible to hang racks of ribs or pork tenderloins and only sacrifice a little meat on the charcoal basket end. I'm biased with my exhaust holes, but I have not used a drum with other styles of vents. 
Seriously, you came to a great place.
