Ed, view cameras have three types of backs and there are roll holders for each.
Graflok, also called international, backs accept roll holders made to fit them. Most also accept cut film holders. My little 2x3 Cambo has one of the exceptions, a 2x3 International back that has no provision for mounting a regular cut film holder, but AFAIK all 4x5 Graflok/international backs accept cut film holders. Horseman, Wista, Arca Swiss, even Graflex ... 6x9 roll holders are available, also a variety of 6x12s. There are some Linhof roll holders that fit international backs too.
Spring, also called Graphic, backs. All accept cut film holders and roll holders that slip in like cut film holders. Cambo/Calumet and Sinar roll holders, for example. The pricey Sinar Zoom will do 6x12 and I think there's a 6x12 Cambo too, but it isn't common; both can be used on an international back. I use Adapt-A-Roll 620s on my 2x3s, don't think that an AAR 620 to fit a 4x5 camera (they exist) is a good choice for you.
And then there are 4x5 cameras with proprietary backs that accept only their makers' cut film holders and roll holders. Plaubel, for example. AFAIK there are no 6x12 holders for any of them.
Re older lenses, visit
http://www.largeformatphotography.info/ and read the FAQs. If you have questions, join the forum and ask there. APUG has much less LF traffic.
www.graflex.org also has a huge lens list.
For modern lenses, visit
www.schneideroptics.com and
www.rodenstock-photo.com to learn about the latest most best.
There are many view cameras around, mainly used, at a variety of prices. Too many to give advice other than "use what I do." The one think to keep in mind is that given your fantasies you'll need one with quite a short minimum extension.
I wrote "fantasies" because I have a 35/4.5 that just covers 6x12. I use it on 2x3, don't often see subjects that need a lens that short. Perhaps I should work on improving my vision, but I think that really short lenses are hard to use effectively. Going 6x12 is one of my fantasies, and I have trouble imagining situations in which I could use the 35 well on 6x12. Understand, though, that when I
do find a situation that wants the 35 I'm glad I have it.