I'd recommend:
1. Trying out the demo, if you really like it (and hey it's absolutely the best grand strategy game ever IMO) then you're going to be a lot more motivated to buy dlcs and or a lot more satisfied with your purchase regardless of whether or not you only buy the base game.
2. If you even have to ask, you're probably fairly interested in it, after trying out the demo I'd recommend buying all the major gameplay dlcs... right now that'd be legacy of rome and sword of islam. Assuming a sale isn't on at the moment then it may simply be better to buy the base game and then go for the dlcs later (even if that later is 2 hours later, they're definitely worth it).
3. After you've tried out all the gameplay dlcs I'd definitely go for most if not all of the other ones. The extra portraits makes the game so much more immersive, and if you like the music then extra music certainly does as well. Don't underestimate the effect that the additional visual/music art adds to the game. Lots of very high quality music can make any game fun for 10-20% longer if not more. It's not exactly like there isn't much stuck into vanilla either, it's not something you need but to me it was a very worthy addition.
4. Sunset invasion can be disabled, probably buy it only if you have all other dlcs... but with that said if you've got a spare 5 dollars and it sounds like it'd be vaguely interesting, well it's vaguely interesting at least. It can be disabled, most forum posters just winge about it because they wish dev time was spent on a more "useful" dlc. You can choose whether or not sunset invasion is enabled or disabled every time you start a game. As people say, you'll probably try the game with it enabled 2-3 times and then get bored of it and only use it every one time in 20. It's purely entertainment, while the other dlcs are both entertainment and improvements in a historical simulator. That doesn't mean it's a bad thing and not worth the money though. It was always designed as a dlc that might not be worth playing, so don't assume that just because that one isn't as value for money as the others that SOI and LOR aren't absolutely top quality.
Sunset Invasion is good if you are not bothered by a fantasy scenario where Aztecs invade Europe and want the western Europe so have some more excitement.
Or "If you want some amusing storyline events and a late game challenge for western europe" is another way of saying it.
