You seem content now, but here is another perspective on the merits of the MKI that coincides with what others have said.
The MKI was introduced in 1987. It was replaced by the MKII in 1990. (So) There are much more MKII lenses and users around. For the price it is an excellent lens, because of this many will sing praises about the MKII. (But) Much fewer have/used the MKI to sing praises for the lens.
I actually had the MKII before the MKII. I got rid of the MKII after getting the MKI. The MKI is much better built than the MKII. IMO it is better built than the Canon 50mm f/1.4, since it is not prone to AF failure like the f/1.4.
The MKI & MKII seemed about equal in IQ which is good (better by F/2.0). I also feel the same about the f/1.4 (better by f/2.0).
Besides build, my MKI seems a bit more accurate at AF in lower light compared to the MKII I owned. But that is my opinion and could be all in my mind; as I never tested that. Neither is as good in low light compared to some of my other lenses. I’ve never heard anyone else say any improvement in AF accuracy in low light, so it could be just individual copy variation.
The negatives of either f/1.8 version (IMO), I like to f/2.0 before sharpness is typically good enough. It has pentagon OOF highlights (so I typically will not use it where that might be a big issue). Bokeh is not all that great in general.
Also some are willing to pay a premium for the MKI. Paying more for it than a new MKII and more than the MKI originally cost. Even though I consider it the better lens, I would definitely not pay more than a new MKII - but that is me.