From oldest to most recent for your 501C
- Distagon f/4 50 mm C Later models are T* coated and marked as such 63mm filter
- Distagon f/4 50 mm CF from 1982 All T* coated, shutter Prontor design, 60mm filter from now on
- Distagon f/4 50 mm CF (FLE) secondary ring for moving elements for better close focus performance (Floating Lens Eement- Get it !! )
- Distagon f/4 50 mm CFi (FLE) i for Improved, interior coating and body design Also an E version CFE later with electrical contacts based on the i design.
- Distagon f/4 50 mm ZV Classic Last series made as the V system ran down not many about were expensive, even for Hassselblad ! From 2006 no electrics may still be available new?
- (The CB series was a "cheaper" range in 1997 no 50mm in that range was made.)
Simply, as you come down the list price will rise, it is of course dependant on condition.
The early C non T* lens gives a lower contrast, that is still favoured by many shooters notably in portraiture.
The early shutters before the Prontor, the Synchro-Compur, can be more expensive to repair and parts are become scarcer, many will argue with that as repair shops vary in the parts they have available so experience varies.
To see a difference in a final negative between any of those lenses you would have to have a "golden" eye IMHO.
You have obviously seen these BUT they are for the F Series (They have no shutter in the lens and cannot be used with your 501C
- Distagon f/2.8 50 mm F (FLE)
- Distagon f/2.8 50 mm FE (FLE)
To answer your questions: There are small difference between them, even the "recent" i range it is hard to see a difference except in extreme circumstances eg resistance to flare.
Avoid any with obvious fungus of course and any hint of a "lazy" shutter, if not fired regularly the lubricants "gum" up and a strip down is required unless you spend an evening firing blanks which can work.
Heavily used examples may be that because they are particularly good!! There is some quality variation but you are unlikely to see it on film unless the lens has been dropped and the elements mis-aligned.
What lens/ lenses have you as the above pattern is common to all the focal lengths, with of course some exceptions such as the Superachromats and UV Sonnars so you should become familiar if you are extending the set?