I had the same query in my mind: wouldn’t it be a form of greed if the main motivation is to seek wealth and happiness when I do these good deeds?
However, I think it is a matter of stages (次第).
I am at a stage now where I need such motivations (like desire for a better life) to keep me practising Buddhism - this is as opposed to not being aware of Buddhism and Karma and going about my own way.
With more learning and guidance from my Teachers, I will hope to reduce and eventually eradicate such impure motivations. My main driving force to practise Buddhism then will be compassion for all sentient beings (菩提心).