A lot depends on the particular kit you're dealing with, and how it's been stored over the years. So far, all of my Pocher kits have been ok... I've never had a problem with the brittleness of the wheel rings that you describe, so all I've ever had to do is use a very small screwdriver tip to push the spoke nipples into place... the plastic wheel rings have always given way nicely. But you never know what you get when you buy a 40 year old kit. You also seem to have had issues with the kit springs, which I never had. Must have something to do with how the kit spent its previous life before you bought it.
But just think how proud you'll be of the finished model, knowing what you went through!
Edit: Actually, the toughest part of building a Pocher wheel is making sure you put the spoke nipple on the spoke in the correct orientation... the wide flare goes down into the plastic wheel ring. I use a magnifier desk lamp when building my Pocher wheels... I couldn't have done it using only my natural unaided vision!