I personally would go about getting this done a little differently than it sounds like you're going to...
I would pick out a great repair shop with a fine track record in brass overhauls (Oberloh, Mid South, Dillon immediately come to mind). I would tell them exactly what I was expecting to get for my money. Either your project will be feasable or it won't and either you'll have enough for it or you won't. This is the only way to really find out how much it will cost.
The advantage of going about it this way (in my opinion) is that you have a "project manager" that will take care of the entire project (in house work, out-sourcing, etc.) and can get things done that the general public can't. I'd much rather have one person manage it than have to figure out for myself who would do my dent work, my plating, my valve job, my fabrication (if necessary), my assembly, and my shipping.
I would guess that the overall to as close to new condition as possible by a great repair person will cost you +-$5,000. I would also guess that your horn could be put into playing condition for far less than that should you decide that spending $5,000 on your sousaphone isn't worth it.