Yeah, the build quality is fantastic - can't go wrong, strength-wise with Russian gear like Zenits. They're built like tanks, quite literally.
Just search for Helios 44m 4 or Super Takumar 55 F2 - or any M42 lens come to that but those are two I really like, with the Super Takumar coming top. You will need an adapter from ebay too, that does M42 lens to EOS body. Most lenses will fit fine, but apparently some can get in the way of the mirror on the camera. I've never had that problem though.
Glass in high-end lenses now will be much better than the equivalent from many years ago, but I still say I can get a much sharper image with my Takumar than I could with a lot of modern lenses under say £400. You just get zero electrical technology and nothing automatic. There will be a manual aperture dial on an older lens, but if you set the camera to AV mode it works fine.
The coating on the Takumar in particular is also very good - again, probably better than many cheap modern lenses, but I do sometimes find I need to bump up the blacks (in levels) in the Canon software afterwards but that's not really an issue for me as I will do it whatever.
Tom