There really is a large leeway here. Although the trailer may seem roadworthy ask yourself if you have the resources to follow through with all that is necessary for finishing the trailer. I know in my case with the same trailer that was in really pretty good condition, that I have just spent in supplies alone almost 2.5 k to do a frame on rear end replacement (new toilet, plumbing, water heater, water pump, new intellicharge with new rear end wiring, new floor replacement, replacing the collector waste valve, refiberglassing the black water tank, etc.). During the last 5 years I have put about another 5k on everything from new brakes (still need to do the axle), new refrigerator, refinishing and rebuilding cabinetry, new finish floor, mattresses, reuphostelry, etc. These are only supply costs since I have done all the labor myself.
So for the price, I would work backwards to see what it is worth to you, figure your budget for getting it on the road while doing it the right way, and then come up with your number that you can afford. If the seller bites and it is a fair number, you will be able to complete the project.
If you need any help with the project, I probably have seen it on the trailer. Feel free to reach out.
1964 Tradewind Double