Greetings BabyStreamer!
Welcome to the Forums and the world of Vintage Airstreaming!
The price on your Overlander was a very good deal unless you uncover something major. I gave nearly twice that for my '64 Overlander in 1995 beacause I couldn't resist its floorplan and it was in so much better condition than the other 30 that I had examined. To get an idea of the typical prices for Vintage Airstreams, you might want to examine the document at:
Price VS Condition for Vintage Airstreams
The Good Year Marathons that I run on my '64 Overlander ran just under $90.00 each with mounting and balancing (225 75 R15 ST - Load Range D). I am on my second set as I replace my tires at five year intervals, and have been very well pleased with both sets - - when traveling, I check the pressure at the beginning of each day and run the maximum p.s.i. listed on the sidewall.
You might want to consider repacking the wheel bearings before travel as well. Especially if the coach has sat for any length of time, or if the date of the last repack is unknown - - it would serve as a rather inexpensive insurance policy.
If you haven't already discovered through surfing this Forum, another issue you will want to be prepared for is rewiring the Bargman connector on the trailer. If your trailer happens to be like both of mine were when I acquired them, the Bargman connector will not be wired to the current industry standard and as such will likely be incompatible with the connector on your tow vehicle. When I picked up my '78 Minuet, I was sure to carry a Bargman trailer end that matched my tow vehicle and was prepared for the inevitable rewire. You can find the wiring color code used by Airstream when your coach was built at:
1966-1981 Airstream Bargman Plug Wiring Information
The wiring information for the Bargman style connectors installed on most modern tow vehicles can be found at:
Modern Bargman Plug Wiring Information
Good luck with your Overlander!
Kevin