The pbf files - to get those into BB Manager its File>Import>Folder and then select the .pbf file. A new folder full of songs will appear in the BB Manager song folders list.
The 4GB card is good to get start. Song files are very small, but drum kits are up to 100mb each. I use a 32GB card. That is the largest that the BB will accept. I have a couple thousand song and somewhere in the neighborhood of 200 drum kits on my card.
All that the BB comes with are basic patterns, just like an old school drum machine.
Personally, I like songs with bass and drums, at a minimum. I often add keyboards. The drum patterns on the premium song sets are good, but many people find them very complicated to use. And, you can buy those songs as individual packets, you don’t to get the whole $200 deal, although, if you like those, it is a good deal. But, if you just want the Beatles or Fleetwood Mac, etc., you might be better getting the individual artist packs.
I like the stuff from the Forum, but I am biased, as I made quite a bit (not near as much as Persist), and what I made I made to suit my taste.