I found myself in a very similar situation. I wanted the game mainly to play with my girlfriend who’s not much of a card game person herself. We played Arkham Horror some times but it didn’t really appeal to her.
L5R was more to her liking. Being an Old5R player and huge fan of the game, I went immediately for 3 cores, so I can’t really help much there.
Having read the Learn to play guide book and watched a couple of plays on YouTube, I found the basics easy to learn and teach. On the few occasions doubt arises during play, we both go online and figure out the correct rule. On a couple of occasions, we just decided for ourselves what seemed fair and only checked the rules later so as not to spoil the fun. I would highly recommend watching one or two videos of how the game is played on YouTube, it really clarifies the rules (personally, I liked the Team Covenant ones).
As for card knowledge, we haven’t felt it to be much of a problem. Of course, we both made rookie mistakes, such as her collecting a bunch of conflict cards on hand and running into my RoB (yeah... almost slept on the couch for that one) or my many blunders against Jigoku players (who were kind enough to put up with my rookie mistakes and help me get better). All in all, to play in a friendly environment, I don’t think you really need to know every single card to have fun. However, as you get to know the cards and get better at the game, enjoyment will increase.
We both have had a lot of fun playing the new L5R, with the rage inducing moments of RoB to the frustration of Voice of Honor to the exhilaration of victory over your crushed opponent (kidding...). We don’t play competitively between us and it’s great.
Hope this helps. Cheers