So you say you are going to build a UDS, but don't want it vertical. Can I ask why? A UDS is in fact an Upright Drum Smoker, and they run circles around horizontal drum smokers-not the huge and thick tank smokers, but ones made from a drum. Reason is, the drums are just too thin to hold heat with an offset firebox type setup, they leak, and you will play hell trying to keep a fire going consistently let alone maintain temp. Ask me how I know this. The only way I can describe trying to cook Q with an offset is like running a steam locomotive. You know the filthy guy shoveling coal into the firebox? That will be YOU! Build a proper UDS(vertical) and you'll spend a lot less time playing Coal Tender and more time enjoying beers and later, good Q. Not trying to be harsh, just saying I think you'll get the most bang for your buck with a true UDS, and it's worlds easier to run especially if you are new to it all.
UDS'-55&16, Akorn, Bar-B-Q Kettle, Drum Grill, miniWSM, cobweb-covered gasser. I'm JD.