I think I'd have to second the ideas of researching what you're buying and to buy off of RFF or from a camera shop / dealer that's going to stand by / properly vet what they sell. As I know all too well from experience, it's all to easy to get carried away bidding on eBay with descriptions of lenses that are "all good", "working fine", "no problems" and so on.
A Canon 35mm 3.5 that I have with dust, haze, some mold & a sloppy aperture ring that doesn't align with it's numbers is kept near my computer to remind me of the perils of bidding on eBay without real thought and research (It takes vintage looking photos though 🙂). I would also suggest that photos of lenses can be quite misleading. This lens looked really fine in the photos.
With CV lenses I've only got an Ultron 35 f1.7 - its quite a large lens and intrudes a lot more than I thought into the viewfinder. No problems normally but you might prefer a smaller bodied lens if you find something of this size intrusive.