A quick search shows most 18,000 BTU window air conditioners are in the
$600 range- not the $1600 you mentioned.

For $600, I'd pop for a new one as it will likrly cost you a good couple
of hundred buck to get that 20 year old one repaired. Also, new ones are
much more efficient and use a lot less electricity

It's through the wall. Those are the expensive ones. I also need to find one to fit the existing opening (27 3/8" by 17 15/16") Frederich makes several units that have these dimensions. So anything I would save on a smaller unit would be offset by the cost of changing the size of the opening.