Getting a ZD for some sort of image quality improvement seems risky. It's one of the least capable MFD cameras you can get.
Worth noting is that it degrades very quickly with increased shutter speeds, multi-second means lots of noise increase if I remember correctly. I've seen raw samples but I don't think I have any left. I would not recommend this cameras considering how little extra you need to spend to get something a little better. In a studio with flash you can get good results though, so it depends on your shooting use cases.
Getting old MFD gear with the idea to improve image quality is not a nice space to be, the gear is worse in almost every aspect than a recent high res 135 camera so you will feel stressed by the fast pace of improvements coming mainly from Sony and Nikon quickly making your MFD falling behind the competition in most aspects.
That said using legacy MFD gear can be fun as it's different in an old-school way, and I think it's there you need to focus. I shoot MFD on a Linhof Techno tech view camera to get the large format photography feel and creative possibilities without having to mess with film. I'm very happy with that but also know I sacrifice DR, long exposure, live view, etc compared to modern cameras.
As I understand the budget is really tight though, and looking for a second hand 1Ds III for your Canon could be an option, which has less banding noise than 5D mark II. Or you do get a 5D mark II, should be real cheap second hand. Yes banding noise is there but it will depend on your use cases if it's a problem or not. I can't remember last time I had banding issues when I shoot with mine as I rarely need to push shadows. If I'd do that a lot then I'd be frustrated with the banding though, but then there's no camera like the Sonys and Nikons with Sony Exmor sensors. Older models of those are available second hand at low prices.
If you really want to go MFD the 22 megapixel backs provide great image quality, no banding noise issues, decent DR etc. I can still shoot those today and like the results. Better than most Canons (with 7D mark II, 5Ds etc their banding problem is gone though). I would get something a little more modern than the ZD though.