I plan on modeling it as if it were new (in case I want to use it for a scene one day). Then I'll add wear and tear to it to make it look abandoned. I haven't decided on what kind of seat I want to model yet as it will depend on the type of wear I want.
I have several reference pics of abandoned wheelchairs so it will be just a matter of how I want the final render to look.
Don't be satisfied with what you can do but rather strive to do the things you can't do!
Exceed Expectations!