Think about what you DON'T have to do here.
Repaint outside - $2000, Reupholster - $800 - $1200, Curtains - $440, Recarpet - $????, Repair wood floor - $?????, put in a new stove - $300, put in a new fridge - $900, repair or replace water heater - $215, put on or repair AC - $700, new toilet - $150 - $200. (I don't think the toilet has been replaced, but so what? My '78's toilet looks just like that one in placement and the '79's isn't much different, just a different model.) The grey tank is not a snap to replace, but it isn't all that difficult. Chances are a LOT better of finding a stock replacement than finding something for your black water tank. Besides, you need a portable tank for the shower anyway, you can get along with a portable if need be and not repair the grey tank. You will likely be in for a heater - $550. That one is 29 years old and I wouldn't trust it without much checking.
Do I know the seller? No. Do I think this would be a nice trailer at a reasonable price for someone? Yes. Will it need work? Probably - show me a house, car, or trailer that doesn't need work of some kind.Will it need tires? Likely. Brakes? Need to be checked. Etc.
1978 Argosy 6 Metre Minuet
2006 Toyota Tacoma