I would guess the decision on whether to replace your axle on a '55 or '56 Airstream is subjective and may depend upon how many miles you intend to pull it and what condition it's in when you get it. I'm sure some are still using the original axle. I chose differently and converted my '55 Safari from a leaf spring axle to a torsion axle. You can see a little of the slowly continuing story of my Safari here: http://www.airforums.com/forums/f338...ari-45822.html
It takes some measuring skills to order the axle and some wleding skills to weld the brackets on, but it is not a huge task. I think the torsion axle will end up being cheaper than new springs and regular axle. Plus I got new brakes with it. I expect the trailer will ride smoother, too, and I was able to adjust the ride height by selecting the appropriate arm angle on the torsion axle. Others opinions may vary.
Regarding wheels, my '55 Safari originally had (and still has) the same 6 on 5.5" bolt pattern as current Airstreams. I have seen '54 Safari's that also had 6 on 5.5". I have heard that some early '50s models may have had 5 lug wheels, but I do not know the breakpoint nor the exact pattern. Maybe that stopped in 1953
or earlier. Or maybe it was only on tthe smallest models (less that 20-footers?). If you are targeting 1955
I think you will most likely find they all have 6 on 5.5" lug patterns.
Good luck, whatever trailer you get and whatever you decide as far as axles and wheels go.