Hello there!
Boy, you sure do have a little work ahead of you! It should be a fun project. I restored a M416-A1 last year and documented my experience. I also powder coated the frame, replaced the rusted out bottom, bedlined the tub, and tire matched it to my jeep. You can read about it here. Maybe it will give you some inspiration or ideas.
http://www.higherground4x4.com/forums/showthread.php?4601-Rebuilding-a-military-M416-trailer/page2
Here's a few things that I found out, and may save you some time.
1. The powder coat company can sand blast the frame for you. Even when I had mine completely sandblasted, they did it anyways.
2. Be CAREFUL when powder coating the landing leg support. The locking pin is NOT removable. If you attempt to remove it, you're probably going to bend it all to hell. It's best to remove the landing leg assembly and NOT powder coat it. Just blast, primer, and paint with Rustolium.
3. I have a LOT of resources and web links that I can share with you on finding parts. Just remember that the M416 came in 3 models. You have a Standard M416. There is the M416-A1 with the hydraulic brake. The drum is different, so the races/bearings are different. The M416-B1 had a totally different axle, as it used different rims/tires. So, in your search, make sure you are not buying parts for the A1 or B1.
This page has some really good reference material and replacement parts suggestions for the M416.
https://sites.google.com/site/m416trailerproject/m416faq-html
Good luck on your project. I hope you will have as much fun as I did!