Am I in the right direction looking at RS485 for connectivity?
Yes.
Should I consider designing an easy/affordable solution?
Yes.
Would this be useful to other people or am I reinventing the wheel?
I hope it would be useful because I'm spending a lot of time (re)inventing this particular wheel 
If you are not in a huge hurry (and I assume not as you are considering designing your own system) you may be interested in this
It's still early days, prototype PCBs have been made but not loaded and a heck of a lot of code has to be written yet but this should give you up to 254 nodes each up to 1200 metres apart with each node being a Mega1294-based Arduino.
Rob