GameXplain just uploaded a video on the Gacha elements of Dr. Mario World.
The game has two separate currencies, Coins which you can use to buy characters, and a premium currency in the form of Diamonds, which you can buy extra hearts and characters.
The maximum price you can pay for diamonds is $69.99, that's more then a full price game.
There's a stamina system in the form of hearts, which based of the top left of the video, has a maximum of five. Unsurprisingly you can refill your stamina using Diamonds.
Character unlocking costs 4000 coins, or 40 diamonds, and it's completely random which character you get, and if they're a Doctor or an Assistant. It's essentially a loot box.
There's also tickets that you can use to unlock characters, you receive one after completing the first world and presumably receive one after completing every world.
You can get duplicate assistants to level them up.
There's a daily reward system. The reward was 200-300 coins, which is "not enough the make any meaningful purchase".
There's a challenge system to unlock more coins, but based off the video these rewards are a similar amount to the daily rewards.