First, this is a great thought. Gamification has been a theme of other challenges associated with behavior as well; we've also studied how positive feedback can help to encourage behavior change when the change is relatively low "cost" but has a high long-term benefit.
I also think it's great to be conscious of people's specific dietary needs. That being said, consumption of fresh produce is a good thing in virtually all cases! :) So I think that for the most part, providing positive feedback/incentives for consumption of fruit and veggies is a good thing. The MyFitnessPal app allows users to customize their daily goal based on dietary needs, and then rewards them for sticking to the goal they designed. That might be something to consider.
Your idea of sending messages via an app about how to better use money is great. The problem I have encountered in the past with apps like this (for example, Mint personal finance app) is that so many transactions are not coded automatically; you have to go through them and categorize them as food, gas, etc etc. it made it hard for me to stick with it. I wonder if there is any way for the user to do something to code the transactions without adding an extra step... Maybe issuing multiple cards tied to the same account, each for a different purpose? Then an app would know, based on which card was used, what the general category of that purchase was.