The core book will get you started and whet your appetite,the other books are just icing on an allready good cake, but who doesnt like a bit of icing?

If you can get them then by all means do so, they will add a little more to the game and give you a lot of extra scenarios .
As this game is so flexible you can pretty much do what you want.
There are a few posts that people have created their own scenarios, you can do the same.
The thing I've found is that if you are playing with a fun bunch of people you can experiment, come up with a scenario to match the scenery you have and play it, if it goes badly pear shaped either change the scenario or just forget it happened and try another

Remember, this is a fun game and if it works for you guys , then nobody is gonna come knocking on your door and tell you you're playing it wrong ..
Good luck
Roger