Greetings Race 606!
Welcome to the Forums!
The 1975 Argosy you have found is from the third year of production. It shares a good deal in common with Airstreams of similar vintages and reapair/refurbishing is quite feasible. If you are thinking of costs for professional restoration services, you can anticipate some of the following as typical replacements on a '75 coach:
New 6 gallon water heater $500 to $600 with installation.
New Suburban furnace $500 to $600 with installation.
New Dometic RV refrigerator $1,000 to $1,500 with installation (depending upon size needed).
New air conditioning unit $675 to $975 with installation (depending upon size and brand chosen).
New Thetford toilet $250 to $375 with installation (depending upon model needed).
New water pump $125 to $225 with installation (depending upon brand and model chosen).
New Univolt or similar power converter $200 to $500 with installation (depending upon brand and model chosen).
New coach battery $100 with installation.
New Dura Torque axles with installation $2,500 with installation (may be higher depending upon motor freight delivery charges).
New tires with balancing $500 (set of four Marathons).
Painting - - my local estimates from quality independent body shops has run between $120 and $150 per linear foot including painting to the original two-tone standard used in 1978.
New OPD valves for existing aluminum LP tanks and recertification approximately $75.00 per pair (may be slightly higher or lower depending upon size - - my 20 pound tanks were $75.00 for the pair this past spring).
Will the coach that you are looking at need all of the above - - it would be highly unlikely in my experience. It will likely, however, need several of the items. My '78 Minuet needed a new water pump, air conditioner, Univolt, battery, water heater, toilet, axle, tires, brakes, bearings, black water tank repairs, new thermostat for refrigerator, new electric element for refrigerator, new 30 AMP electric supply cord, thorough cleaning and minor repairs for Suburban furnace, and thorough cleaning for refrigerator burner. On the other hand, my '64 Overlander needed a new refrigerator, furnace, air conditioner, water heater, Univolt, battery, water pump, water tank, breaker box, new 30 AMP power supply cable, and new LP tanks (it had been separated from is Worthington Aluminum tanks prior to my purchase), and new break away switch.
The price at which the trailer is being offered is such that you could easily justify rather significant expenditures for repairs/refurbishing/restoration. I paid about twice that amount for my Minuet and several times that amount for my '64 Overlander. To get an idea about general values for vintage rigs, try consulting the link below:
Vintage Airstreams Price VS Condition
Good luck with your decision!