Hardwood sanding

I am about to finish installing 1200 sq ft of mixed red white oak in the cottage.

The hardwood is of utility and rustic grades and contained black knot holes. What do I use as a filler ? Same color? Apply filler first or after sanding.

Some boards were splits and have fine open lines 1/16 to 1/8 at most extreme, what do I use to fill these in?

The hardwood was unfinished, what grit will I be starting with? I have been reading 24-36 but those are for finished floors.

How many belts should I get? And generally what are rental rates?

Thanks for tips from the experts as always.
Tina