Will this hold my weight during extended work sessions?

Yes. It's rated for garage use and designed to support a mechanic's weight. Test it with your full weight before committing to it as your primary seat for eight-hour days — every garage is different.

Do the casters lock, or will it roll away while I'm sitting?

That's a good question, but the locking mechanism isn't specified in the product data. Before you rely on it for stability, confirm whether the casters have foot brakes or swivel locks.

Can I spray-paint the red finish, or does it need prep work?

If you want to change the color, light scuffing with 220-grit sandpaper will help new paint adhere to the existing powder coat. A primer-first approach is safer than spraying directly over it.