Dishonored is very good without the dlc?

There's 2 story dlc's, the game itself is pretty big, but if you want some extra fun, the dlc's are good too.

Daud is funnier to play than Corvo, but the base game is worth for sure.

The 2 story DLCs are entirely separate from the base game, and nothing is detracted from the main game if you don't play them. It's an alternate story based on one of the characters in the main campaign. The challenge pack is a series of time trials, etc, and offer achievements and leaderboards. The Walker Arsenal DLC contains all of the pre-order stuff which is just some charms, extra money, etc, and isn't required at all.

DLCs aren't necessary. But if you can get the complete edition, I say go for it.

Also, if you do want to get the DLCs eventually, get the GOTY version now instead of buying each DLC individually later, because you'll end up paying more. Bethesda do not like to put their DLC on sale for anything lower than 50% when a GOTY edition is available.

Keep in mind that Bethesda is one of the greediest bastard publishers out there. If you plan to buy the DLC later you'll always pay much more than the complete pack (GOTY, Legendary or whatever else they may call this) costs. Right now for example all three Dishonored DLC cost €12, but the GOTY edition with the base game and all DLC only costs €8.5. That's quite ridiculous. I bought the game itself a while ago but I won't buy the DLC because of this scummy business model, even if it's just a few Euros.

That said, Dishonored is a great game and two of the three DLC seem to be pretty good (story missions that take a few hours to complete), so get the GOTY edition now.

The base game is great, the DLCs are good addons that you play besides the main campaign. I only played a part of The Knife of Dunwall, and it is nice. Some say it might even be better than the main story, but of course way shorter.

I loved the original game and I loved the story DLCs. They are not necessary, but great if you want to see the whole story from different perspective.
I didn't play the Dunwall City Trials though. It seems pretty boring.