Do I need bolts or hardware to mount this to my hitch?

Your existing hitch receiver hardware will work — slide the bar in and use the fasteners that came with your hitch. If you've lost those, any stainless steel bolts rated for hitch duty will secure it tight.

Will this step bar fit a 1.25" or 1.5" receiver?

This is designed for 2" receivers only. If your hitch is a smaller receiver size, you'll need an adapter or a different step bar model.

Can I paint or powder coat this myself before installing?

The black finish is factory applied — it's ready to mount as-is. If you want a custom color, you can strip and refinish it with high-temp automotive paint after install, but start with a clean metal surface and follow paint manufacturer instructions for prep.