One of the best things about most medium format cameras with the exception of perhaps the Pentax 67 and the Mamiya 7 is detachable film magazines. With multiple film magazines and the ability to switch magazines at any time you desire you are given a...
Generally, yes, but a hundred dollar entry-level Pentax lens might not be up to the quality of a $400 Sigma "Pro" lens. As a rule, if you compare lenses that are competing in the same market segment, the Pentax will be better.
Most posters that you've seen up to now (i.e. landscapes, mountains, beach scenes, etc.) were probably shot with medium-format cameras like the Pentax 67, the Hasselblad cameras, Mamiya 7, etc. or more likely, large-format cameras, particularly 4x5 f...