Welcome to the forums and the delightfully challenging world of old Airstreams!
Depending on how many of those weekends you're planning to work on the Argosy, and how many hours per weekend, I think you may be conservative on the time and perhaps a bit optimistic on the money.
Before you trash out the entire interior, think a bit about what you can do to change the way some of the existing stuff looks. If you're not into the 70s thing, you'll probably want to get rid of all the upholstery, but depending on how your interior was done there may be some updates you could do for a more modern look without having to build custom cabinetry. The PO of my Argosy (trailer) put some very nice wallpaper in place of some of the woodgrain plastic veneer, but left enough to get the woodgrain feel without it being overwhelming. I'm pretty happy with the results of that, and especially with the fact that he changed the formica to something a bit less "avocado" than the original color.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill