I have a '77 Safari, The Safari is on of the most valuable and desirable trailers because of the length and weight.
There can be a very wide range of values on trailers based on condition.
A fully restored, polished trailer with new appliances and axels could be as much as $20,000.
A trailer that need a restoration could be as little as $2000 depending on the extent of work required.
When I bought my Safari I had a budget of $10,000. There was nothing obviously wrong with the trailer so I subtracted from my budget 50% of the total of all of the items the trailer may need that I couldn't verify worked or would be reliable long term, I assumed that at least 1/2 my identified potential repairs were either not needed or not needed right away. I made an offer based on that figure.
On a trailer of that age I would have the following concerns:
-Appliances - Fridge, Stove, Water Heater, Converter, A/C, Water Pump, Furnace
-Waste Plumbing - a 73 won't have a grey tank
-Fresh water Plumbing - I replaced my plumbing with PEX
A lot will depend on how you plan to use the trailer and how much work you are willing or capable of doing yourself.
If you camp mostly in Parks with full hookups in the northern states, the lack of grey water tank, or functioning AC probably won't be a problem. If you plan to Boondock in the southern states these will be a priority.
Get a flash light and check the floor in the following places:
1) stick you head inside the under-sink cabinet in the bathroom and look for soft floor or holes behind the shower and under the bathroom counter.
2) Under the bed(s) in the bedroom and behind the couch at the front - look by the wall for signs of water leakage and check with a screwdriver for soft floor.
3) stand on the rear bumper and bounce up and down, the aluminum shell should move with the frame/bumper, if there the frame/bumper move independent of the shell you have a problem.
4) try to see everything operate that you can, furnace, AC, water heater, water pump, converter, open the awning, flush the toilet, open the awning, inspect the tires for age/wear, inspect propane hoses for cracks and tanks for dates.
Good luck - the Safari is a great trailer!
1977 Safari Land Yacht
2005 Toyota Tundra SR5
2010 Ford Flex Ecoboost