Canon and Nikon 35mm SLR's would be a good choice because you can find not only good used cameras, but good used lenses for them ... the added feature is that if you ever do purchase a DSLR, all the lenses you have will fit their matching DSLR body.
I bought a used Nikon N70 a few years back. It was (and still is) in great condition. I paid just under $500 for it. (Note: Mine is not digital, it's 35mm.) So I would say around $300 if it's 35mm.
Yes. There is a Holga 35mm adapter. It is available new from the International Lomographic Society shop (see link below for local distribution arrangements). You will probably need to find a specialist developer to get the full effect of having the i...