Fortunately, the Voigtlaender holder proportions seem to be the most common ones. Kodak/Nagel holders will also work (their combination film/plate holders, which have a built-in spring-loaded pressure plate instead of a separate film sheath, are the best holders of the plate-camera era IMHO). I've been gradually accumulating 9x12 holders in that standard for some years and haven't really had too much trouble finding them. Sometimes they come with a camera attached, and that's how you end up with a cabinet full of plate cameras.You can still get 9x12 film but you will need the proper holders with film inserts. These can be hard to find and costly. I would recommend not buying a 9x12 camera to use with sheet film unless it's sold with holders (including film inserts) or the proper roll back.
The Avus itself is a perfectly good camera---it's a dark box and you stick a lens on one side and a film holder on the other, and as long as the bellows is intact all should be well. The Skopar lens is a good design, they were quite well made, but they're around 90 years old at this point and have had lots of time to get scratched or foggy, for the shutter to get broken or sticky, and so on. But I've never heard of an f/6.8 Skopar, and I wonder if the lens has been replaced with something else. When you get the camera, post the fine print around the lens and someone will undoubtedly know what it is.
It shoots basically like any other large-format camera---focus on the ground glass, swap in the film holder, pull the darkslide, cock and shoot, then realize you forgot to stop down and have to start over---but has some of the feel of a medium-format folder, in a vague way that I can't really put my finger on. You could scale focus instead of using the ground glass, but there's no DOF scale and the focus scale is kind of a fiddly little thing, so I think it would take some getting used to (I've never done much of it).
It will come with a pack-film back; they always do. It should be possible to convert these into ground-glass backs or something, but I don't know anyone who's actually done it; everyone in the plate-camera world has a million pack holders piled in the back of a cupboard somewhere, convinced that someday a use for them will turn up. Anyway, that's what the strange hinged back with no ground glass in it is, and you might as well choose your cupboard now and put it in there. (No one ever throws them away, including me; I don't know why.)
9x12 is actually a pretty convenient format. The cameras are fairly small, the film is cheap as sheets go (albeit with a limited choice of emulsions; Fomapan is by far the most common, though Ilford does cut 9x12, I think only for the European market), and you can contact print for a 3.5x5 frame. It's a good way to enter large format, I think. The only real unknown is the lens; if it's a dog, you'll find that out quickly.