Question about purchasable Modules

Quite simple actually: Do they open a new module-specific campaign or give me the opportunity to do that with everything in the module already in the journal or do they just give me a bunch of items (to use via import)? I consider buying one or the other, but I would like to know that one first.

The former.  You build a new Game with the Module as a base.  The Game comes into existence with maps, characters, journal entries, decks, tables, lighting etc. just as if you spend 300+ hours setting it up. Addons are similar to modules, but are fairly small in scope, like a collection of creatures, a deck or two, a collection of pre-setup maps.  They are intended to just add on to an existing game, and can be added from the Game Details page.

That sounds good, thank you. Will definitely buy around a bit when I find more time to GM around (my list of future games is too long already). :D

Oh, one thing more: How long are the sessions officially estimated when a Campaign Module is 10+ sessions? "Session" is quite a vague and unspecific measure for time. Because 14 sessions of 8 hours each is impressive, whereas 10 sessions of 2 hours each is less so. True, it depends on the players a lot and how fast they progress and how much they sidetrack, but what is the average here? I ask this because I have seen both groups of players who play around a lot and not get much done in 7 hours and groups who power through and achieve way more in 1 hour than the other group in 7 (obviously with the downside of no character interaction, roleplay or anything). For the first group here paying 50 dollar for a module would be a treat in cost-time-ratio, whereas with the second group it would not be worth the money due to being over too soon and the subsequent need for new content.

For the reasons you state, it's really impossible to give a definitive answer for that. You'll need to rely on your experience with your group and your experience in running adventures with different groups. It's an estimate based on the writer/producer of any given module. If you are looking at the WotC modules, you can compare them to the printed book if you have access to one. That might give you a better idea, even if you just spend 5-10 minutes flipping through one at a friend's house or your local game store.