Wow, you sure have your work cut out for you. Good job so far.
I'd recommend throwing out whatever hasn't been used in 10 years. And throw out all old paint and fluids. That stuff doesn't last forever. Then I'd divide up everything into categories like boat/auto maint/welding-fab/recreation(bikes golf clubs et.)/landscaping care/ etc. If possible put your lawn care in an outdoor shed. Once you figure out how you're using the garage space then you can plan your lighting. You can't have too much light.
I wouldn't worry about painting the walls and floor until you get organized and light the space properly.