As a general idea it's a good one, but when you start looking into details many problems will show up on the part when paying people to refill it.
For example, how would you make sure the user did put water and not mess up with the sensors? even if you completely seal the machine in a secure way, how would you make sure the user did put good clean water? same would go when refilling all other ingredients.
Also, consider the network delay and fees, you'll need to use cryptos that have near instant confirmations so the user won't have to wait long for the coffee.
Finally, consider the price fluctuation, whoever sets up the machine will have to accept such risk unless they're believers of the coin.