This bike is a great base for cummuting. However if you want to do this for fitness, I recommend a road bike or possibly a cross bike. Most of us have different bikes for different purposes, and havig a single bike for all things forces a lot of compromises and keeps you from really fullfilling you wants.

Concerning your Green Giant. a chain costs $20, chain tool cost $20, Youtube and ParkTools videos are resources common bike repairs. If needed replacing a shifter cable is not really that difficult either.

Cycling can be a very expensive sport but being minimalist you can fun and creative ways to make the parts work.

Also, find local bike co-op. Co-ops are non-profit centers run by people that know how to fix stuff that you are having. They generally have spare parts lying around. In return they ask you donate a few bucks etc.