I supported the body for a month on just the 2x4 frame you see in the pictures. It was more sturdy then you would think. Make sure you put a brace across the door opening as well.
The springs are a odd length and you are not going to find them off the shelf. Going to a 3500 lb spring might be a bad idea. It will pound the trailer rather then flex and absorb bumps. The 3500lb axle is just fine however.
My axle appears to be in good shape. The rust on my frame was not very bad at all. Just rust on the very last cross member that required it to be replaced. I was very fortunate in that. So far I have not found any reason to need to change my axle. If I do change mine I will by a off the shelf axle and if I need to adjust its width I will will cut it in the center and sleeve it. I may go with a straight axle with no drop as well. Get a little extra clearance under the tail that way.
Our Coach is a 22ft Caravanner. The Safari, Flying Cloud, Falcon, Custom and the Caravanner are all built on the same 22ft platform. The difference is interior lay out and window placement. The Safari and Cavanner in some years do run the full street side window arrangement.
Here is a great site with a VERY extensive photo archive if you have not found it yet.
www.vintageairstream.com
The shell is much lighter then you realize. A sheet of exterior skin 4x12ft is only 22lb. 2 people could handle the 18ft weight wise. its the awkwardness of the size thats the problem.
Ohhhhh I have devised a trick for the belly pan. I used some 1 inch x1/8 bar and ran a couple strips to rivet to. The sides that are visible end just under the coach and I have four panels that are removable without having to get under the edge of the shell. I will try to get some pictures today. If it doesn't rain I am hoping to put the brake wires in and the Umbilical so I can get the front pans up.
The question of rot prevention has come up a few times. Most of us that were posting in the Full Monty all sealed the new decking with a Epoxy resin of the variety you would see used in Boats. I bought what I used at West Marine on US 41. I also put down Sheet Vinyl flooring before I put the U channel on. The hope was to seal it edge to edge. So far so good.
The thought with the sheet flooring is no seam and it should make a gasket when stuff is bolted down through it. The main areas may eventually get a laminate floating wood floor down over it. With two young kids It may be a little while as this will be easy to keep clean.
Have to shoot me some direction to your place so I can swing by one day and take a peak.