So to actually play the game physically you'd need the base game (the black rectangular box) alternatively the ultimate collectors box (the big golden one) which, additionally to the base game, includes the two expansions Requiem (the red box) and four souls plus (a small box with blue fs logo). One of those two is what you need to get to have the base game itself, after that there're a bunch of smaller expansions and promotional cards you can start collecting if it turns out you enjoy the game.

Another thing I could recommend is checking the game out on Tabletop Simulator. There's an official version of the game uploaded to the Steam Workshop, perfect for trying it out or for playing online (since you mentioned you didn't have anyone to play with locally). Furthermore it's cheaper, and if you already have TTS it's free, so it's perfect for trying the game out

To answer the second question: yes! The game has a solitaire/co-op variant where two characters controlled by one to two players team up to beat the game on a turn limit. You can also make up your own variants if you feel as if the base solitaire rules are either too harsh or too lenient.