Vintage Sub question

[KFP55]

I’ve started looking at acquiring a 4 or 5 digit Sub (no date) and it a seems like prices are all over the place. I’ve fallen in love with the creamy patina on a few models I’ve seen online, but it appears to not really follow any specific timeline. And some from the 80’a are more “patina’d” than some from the 60’s. I don’t care about the collectibility (error dials, etc), just want the best bang for my buck for the vintage look. I’d wear it, not keep it in a safe.

So...which references should I prioritize and what should I look out for? Would have to assume a 30-40 year old model would have had some parts replaced over time if it’s been service properly. Assume box/papers are not worth it given the premium? And I’ve heard the pre-sapphire crystals are much more fragile than today’s - what year did they switch?

Thanks for the direction - starting the journey and any info is helpful!