Hello Irish, nice to meet you. It sounds like you were lucky in your Overlander find. The Overlander is one very popular Airstream. It is an ideal size for comfortable traveling. And I read the 1965
trailers are popular as they still had "real wood" cabinets inside. Your trailer and mine have the same "airliner" bathroom where Airstream got creative with fiberglass. The sink is over the tub, and the toilet is hidden in the "window seat".
My 66 Trade Wind is one size smaller than yours at 24'. Mine is a twin bed and front gaucho layout. I thought mine was is in reasonable shape when I bought it, but come to find out is had many more needs than I anticipated.
You should start with a thorough cleaning and analysis of your Overlander. Start with the tires, then brakes, shocks, axles, belly pan, frame, and the all important subfloor. I found my subfloor rotted out beneath the toilet. Then assess the plumbing, electrical, and appliances. Check the weatherstripping around windows and doors. I'll bet you will find your 50 year old camper has lots of needs. In my view, there is no sense in making new curtains or re-upholster the gaucho if your axles need replacing. Get your Airstream safe and sound, and then you can decorate.
Check out the Airstream Knowledge Base section of these Forums. Find it near the bottom of the forum page. Then find Overlander, and click on your year range. There you will meet all kinds of nice folks with old Overlanders. You can read about their trials and tribulations in getting these vintage trailers back in action.
Have fun and keep us posted. Here is my Trade Wind as found. I've got over 1000 hours in it so far. Keeps me busy and broke.