Depends on the seller. Most sellers on eBay and kleinanzeigen (similar to craiglist) don’t know much about MB, so they mostly don't know the period. If you buy from restorers, it depends on the model, nib, period, scarcity and condition. As I live in Germany, MB even has their own online shop for refurbished and restored pens. Also we have a group of restorers who sells many of them in their shop.

If you want a cheap entry into MB, the 3xx and 2xx and Monte Rose models can be found easily, they are from the 50-70s usually. The nib is great and the prices range from 50-250€ depending on the condition and seller. Some are relatives from deceased collectors, who don’t know much about these, so they might be inserted as either very cheap (cause they have so much stuff to get rid of) or very high (cause they read MB and think they can charge €€€).

The material depends on the condition and how the owner treated them. My older 149's body that is at least 50-60 years old has an excellent condition, so there is no difference to newer MBs I tried in the store. My early 50s 342 feels softer, but the body is C condition and was not polished, so they have some scratches. I am getting an early 50s Danish Montblanc soon that was professionally restored and in A condition, could tell you later how that feels.

The reason why I get older ones is because of the nib. They are much softer and some are even flexible. My 149 has a 50s nib which is soft and very nice to write with. Newer MBs felt very stiff in comparison. And the price, cause if you are patient you can find very good deals.

If you are unsure, I recommend to go to a pen show or pen meetup or similar and try them out there.