I don’t collect Vintage postcards myself. Since @ Ceres1849 answered many of your questions, I will give some thoughts on other matters. Of course, what I will say is quite subjective. But as a buyer / seller/trader/collector of picture postcards for 40+ years, I think I have learned a thing or two.
In the strict sense of the term, Postcard Collecting involves the picture side of the postcard. The beauty of Vintage postcards is that you have a huge array of subjects on them. From their early beginnings, people collected postcards on their travels, put them in albums and placed them in their homes for people to look at. With the advent of photography, this practice started to slide. Postcard collecting pretty much has been broken down into two areas: Views and Topics. Views are pretty self-explanatory. Topics could be anything: maps, animals, sports, etc. Sometimes an addition to a card can change the type. For example, you may have a trolley on a street (a view) Put Teddy Roosevelt in the Trolley and now you have a topical card. Perhaps the biggest draw of Vintage cards is that they display views that will never be seen again. Small towns are a good example.
I myself prefer modern postcards, really those published after 1990. The photography is much better, the images sharper. As a former stamp collector, I really like it when other members or travelers randomly put large stamps on cards. Moreover, some countries use neat pictorial cancellations when processing mail. Most of the messages can be bland. But once in a while, you come across some interesting ones. There was one card written by a UK traveller sent from Portugal. Two or three days before, the writer noted that the dictator at the time (Salazar) had just died. Where do I buy my cards? My US cards I purchase from a wholesale source in Indiana; I just purchased a lot of 1000 postcards from him. With International cards, I may buy some; I trade for quite a few. On occasion, I have bought collections from those members who have left Postcrossing.
Whatever you collect, try to get as much enjoyment as you can from this wonderful hobby.