Hello Gary -- Welcome to the Forums! That seems to be pretty close to other prices I've seen on similar units. That era of just sub-8' wide units are fun and easy to tow. It's a little tough to tell you all of what you will face but I'll bet it's a lot more than replacing window gaskets & roof vent seals.
- Period furnaces may not be safe but aren't the most expensive to replace either (carbon monoxide). You can check out some basic costs by looking at campingworld.com. The major systems on any RV have stayed about the same modular size/capacity over the decades though the plumb-in points will change somewhat.
- HW heater? Check it out -- not always easy to do unless the owner has been actively running water thru the system -- look at the tank up front -- if it's dark it may be full of algae.
- Air conditioner -- find a 30A outlet (campground) and try it out. Never use a 15A-30A adapter to run the A/C; that's hard on the compressor.
- Fridge -- original? You'll want something like the original 2-way (110V or LP gas) for best options while out camping. This can be one of the more expensive replacement items. It's also difficult to check out -- it can take 1-2 days plugged in before it will really get cool (possibly at the long end of that time if running it on LP). It may work on one and not the other -- we have a lot of folks who have worked on these, though miracles aren't always possible on a 'mostly dead' fridge.
- Black tank? Has the unit been in use? What was left in the tank the last time it was used... ulp, how many years ago? How about the waste gate & seals on that?
- Axles -- You may get by bringing it home or doing some local travel, but will definitely need to see if they need replacing before you'd want to take on any long voyages.
- This is a 30 year old unit -- expect that it may need some floor repairs. Mine did under the water tank in front (window leak), under the battery compartment (seal under tray bad), and at the rear bumper -- very common but hard to see with rubrail trim low on rear platform, but old caulk back there has likely aged. If it smells musty inside it more than likely has some floor rot issues somewhere; this isn't a deal killer if you are up to pulling everything apart but can stretch you out to 1-2 years before you have most of the systems put back together.
Until you really do a good autopsy on a purchase it is not possible to guess whether $4000 on repairs is realistic. It may be close if you're good at DIY -- but will be very far off if you'll need new axles and either an A/C or fridge. If totally fixed up I've seen asking prices close to $10,000, so the purchase price + repairs may exceed that. It just depends on your values, how you want to travel, etc. The other side of the picture is that you'll have a really cool Argosy when you're done -- and just about none
of the other RVs (SOBs - 'square old box' or 'some other brand') built back in that period will be competing for oohs and aahs when you roll into a campground!