@honest-chocolate: I've been using a drum roaster for several years and haven't noticed any fat buildup. I'm roasting whole beans--maybe nibs would have this problem.
You'll at least need to have a low enough drum speed to not break the cacao, but adjustable drum speed isn't absolutely necessary. That being said, you can affect the roast through modifying the drum speed, so adjustable drum is preferable.
Stainless steel is not a necessity. I did a lot of research into drum material for a roaster I'm currently building and most of the roasters I looked into use a mild steel drum. Apparently, stainless does not transfer heat as well as mild steel.
To your last comment about roasting at a low temp, I'd modify that to say 'at a lower temp than coffee'. I do not subscribe to the 'always roast cacao low & slow' philosophy. It works well for some beans, but very poorly for others. With regards to a roaster, you're going to want as much temperature/energy input control as possible.