"By early next year" is a tall order for a 1970's motorhome to be ready. Many new owners spend months/years getting these units back to where they are even close to ready for a long trip much less a Florida to Alaska trip.
I highly suggest you buying a unit that is already roadworthy and the owner has taken fairly long trips. You'd hate to get 1/2 way across the country and have to replace a radiator or even worse, an engine.
If you are firm on the 70's, you may want to consider a GMC motorhome. There are two shops in the Orlando area that specialize in GMC units that you could get serviced over the winter and ready for your trip.
A class-C with a van cab might be a good solution. If you have any service issues, the van cabs are more serviceable by the average shop than the class-A motorhomes. Should you break a windshield in Alaska, the class-C would have an easily replaceable windshield.
If you really want an Argosy 20 to 26 motorhome, you better get started NOW and make sure you get it mechanically sound by the start of your trip. Don't bother painting it. I have heard that Alaska is rough on the exteriors of vehicles.