Looking to purchase SMP

I am looking to add an SMP 300m to my collection. Purchasing new is a little out of my budget, but
am looking to spend 2-3k on a used one. The issue is I don't really know much about how the models have changed over the last 20 years and the subtle differences. Most of my knowledge is about vintage omegas from the 40's to the 60's. I have a pretty good idea of what I am looking for- most likely a ceramic bezel and blue or grey dial. Can anyone direct me to where I can read about what to look for when making a purchase? The Speedmasters have a bazillion models but they all look the same to me over the previous 50 years. Is it the same with the Seamsters, or are they virtually the same?

Also, I have a small wrist and cannot wear a 41-42mm watch, but I'm concerned that the 36mm variant will appear just two small. I have a 38mm Seiko dive watch that looks a little funny at 38mm because of the large bezel and small dial- it wears like a smaller watch.

If you can't wear a 41-42mm watch, then the midsize Seamaster Pro is worth a look. I've got small wrists (a bit over 6in diameter) and I think it fits perfectly. I agree that the thicker bezels of dive watches do make them wear smaller, so you might need to try on a few to see what works best for you. I've included a photo for reference:

This one is a 2551.80 which was in production from 1993 to 2003. Powered by Cal 1120. Early examples had tritium dials. Mine is a later example with Superluminova. The next model which came after this had a coaxial escapement Cal 2500.
Good luck with your search.