I think everyone is overthinking this project. The Op has repeatedly said he's going to use this trailer.
1. As for the lead in the paint, forget about it. Your ancestors lived around lead paint everyday. No worries.
2. If you are set on sandblasting, take it intact and let them blast it. If you do that I'd wait until the roads are clear so it comes back home clean and ready to paint.
3. Forget about lining the bed. You're gonna throw wood and whatever else in it. Shoot paint and use it.
4. Powder coating,,,, forget about it. Once you start using the trailer you are going to chip, scratch and ding the paint. You can't spot fix powder coat. Paint the trailer with a common implement color that you can buy at your local hardware or tractor supply store. Then in a year when you want to touch it up a little you go to the store and buy a spray can of the same paint.
5. Focus on the fact that you are going to use this trailer. First thing that's needed is the hitch needs fixed. The added drop in the hitch appears to be WAYYYY to much. The trailer came with a straight tongue with a pintle ring. I'd use a ball hitch, but that's just me.
In the end you have a very durable trailer that you cannot wear out.