Depending upon year and if the coach is actually 20', you may be looking at a Minuet. The Minuet is a bit out of the ordinary in that its floor width is approximately 6" narrower than the standard Argsoy line. It was introduced sometime in 1977, but wasn't greatly publicized until the 1978 model year. I own a '78 Minuet (6.0 Metre or 20'), and it is a wonderful little coach.
The points that you mention are quite valid for your inspection. I might suggest to also check the condition of the Dura Torque axle. There is a good description of a precise check (measurement) as well as visual inspection information available at:
Dura Torque Axle Inspection Information
As you probably discovered in reading other posts regarding inspection, unless there is concrete evidence that the tires are less than five years of age and that they are properly rated ST tires, you will be in the market for new tires. It has also been my experience that shocks will also likely be on your immediate replacement list as well.
Some other items that you might want to inspect include: the termination cap/drain valve assembly for the gray and black water tanks - - especially if it is a Minuet - - this cap is found oun the bottom of the trailer pointing down toward the road in the streetside corner - - if the trailer has been towed with a hitch that is too high or the owner hasn't been careful with steep drives this valve assembly can be damaged (I know as I have been the culprit with my own trailer). You will want to be sure that the owner can demonstrate that each of the major appliances work (I have included approximate professionally installed replacement costs in parenthesis) - - furnace ($500), refrigerator ($1,000), air conditioner ($700), water heater ($400). It seems like one of the most prolific points for water system leaks on the Minuet is at the city water entrance in the rear curbside corner - - the fitting that goes through the wall seems to have the habit of failing - - it was a problem on my coach as well as three others that I have seen.
You will also want to pay particular attention to the front "wing windows" that are either side of the window over the hitch. These windows can be quite durable, but costly to replace if broken. If my information is correct, the only replacements available new are tinted, and the ones on coaches of the 1970s were typically clear. The side windows while not cheap to replace, they aren't as likely to pose problems.
The price that you mention is in the mid-range for the coaches that I looked at prior to purchasing mine. Last August when I purchased my 6.0 Metre Minuet, the asking prices ranged from $1,000 to $4,500. The $1,000 coach had fewer options (no air conditioning or curbside awning being the major options that would have added significantly to its value in my opinion), and it needed new upholstery, drapes, and floor coverings as well (the mechanicals, however were all funcational). The $4,500 coach was basically road ready with everything being quite presentable but far from 100% perfection - - probably about 75% on a 100% scale.
One way to identify the trailer when you are inspecting it is to look at the serial number. The serial number should decode as follows:
20 - - Represents the length of the trailer as sold by Argsoy
D - - Represents the sleeping arrangement D=Double,
9 - - Represents the last digit of the year of Manufacture
V - - Represent Versailles, OH production facility
#### - Represents the sequential production number
If the first two digits are 20, 22, or 24; and the digit following the sleeping arrangement letter is either 7, 8, or 9 - - you are likely looking at a Minuet. So far as activity here on the Forum, the rarest of the Minuets seems to be the 24' variety that is the only one to have a tandem axle setup - - the 22' variety has an extra window on each side that really increases the interior light and perception of roominess. Whether Minuets were made after 1979, I haven't seen anything posted about Minuets manufactured after 1979 but that doesn't mean that they weren't produced beyone 1979.
Good luck with your inspection!