The game (updated to 1.08 or whatever is the latest version when you ask yourself this question) without the DLCs contains anything you need to play in the west. IF you want to play muslim as well, get sword of islam, if you want to play orthodox rulers, get legacy of rome. sunset invasion is for those interested in a completely alternative, wacky scenario. I didn't get it either, but I bought all DLCs (including songs, portraits and so on).
Basically, even if I didn't wanted to play muslim or orthodox (and I did play a lot of byzantine empire scenarios even before LoR), I'd still want the main DLCs: SoI and LoR - because I want the AI to generate outcomes including those specifics included only in the DLC (muslims with multiple wives, better religious mechanics and events in the orthodox world, and others). The rest, I bought just to support their development and inclusion in the original game from now on.