If you filled any holes you have to put primer on those spots or you will see them when you're done painting. Sanding will not make the paint finish look better, just paint it.

I go to the dollar store and buy that blue colored toilet bowl cleaner. Squirt it on the grout. Scrub with a hard bristle scrub brush. Rinse with clean water and looks good as new. Shocking and a heck of a lot cheaper than grout cleaner.

If you don't put up vapor barrier, you'll always be thinking about it. One chance to do it and the time is now

We used pallets for the floor and walls, left the front open with a tarp that hangs over the open front and put plywood in top for lean-to type roof and had spare shingles so put them up. Been using that for almost 10 years. Good air flow... kindling in one side wood on other. Pallets around us, free for the taking. Most manufacturing plants give them away. So other than a couple pieces of plywood...free.