Hey Karst! Seems like your wife and you like the same games as the missus and I.
(mine eventually got tired of Dragon's Crown, sadly).
The DLC is amazing value and 100% worth getting, but you might want to wait until you're farther into the main game (I wish I had). I believe new characters unlocked are set at the level of your highest character (anyone correct me if I'm wrong), so it's actually in your best interest to level up him as much as possible (ideally 100, the cap without DLC) before unlocking any of them. Since the DLC includes additional adventure mode maps (easily tripling the game time or more), you won't be starved for content to use them afterwards.
On the other hand, DLC characters an weapons automatically have their level 2 and 3 versions unlocked, so that will provide you a significant boost at the beginning. For me it actually unbalanced and trivialized things until the game expected me to have better weapons, so it could be seen as a minus as well.
The final reason is that you will appreciate the new chars and weapons more when you've been playing for a while with all in the core game unlocked. Otherwise they're just "more unlocked characters" among the plethora the game already has.
EDIT: Also, remember the Hero pack includes everything except the 1$ costume packs. It's slightly confusing in the store: just saying so that you don't get one of the 7$ packs individually by accident.