Tex.
Twenty years old is not old at all for most pianos but it really depends on its environment and care throughout its life. Weber IMHO is a good piano. Not sure about ones from 1989, but I've played some 60-80 year old Webers that really perform nicely. They are not the best of the best US built pianos, but they are well built work horses of good quality and craftsmanship.
I restrung a 1988 Steinway S (5'1") with a registered tech that had rusty strings and pins. The piano was hardly used at all, however, the owners lived across the street from the beach. The salt air really took its toll. We also changed all of the flange pins too as well as the bushings.
Got it for a steal, sold it for a great profit. When we were done, it was as good as new.
Salt, smoke, airborne kitchen schmutz can all have an affect on pianos and their action. It all gets to the metal and felt parts, making the strings rust, actions stick, permeating the felt bushings and getting to the flange pins.
Some pianos live in a very clean environment and look virtually new after 20-30 years. Stable temps and humidity, lightly played in its parlor away from the kitchen, a/c and big windows and radiators. Even the hammers are still white. A preowned buyers dream.
Light surface rust on the strings and pins do not necessarily mean the piano needs strings or are brittle. I've played pianos with strings way older than that that still sound good and don't snap from tunings. Most of the pianos I tune are much older with original strings. If they were that brittle, I'd either quit or spend my life splicing broken strings.

Its just oxidation on the surface. Touch a new bass string with an ungloved finger and the salt from your hand will stain the copper. Its really not a big deal. Of course, no one here can tell what the extent of rust you're talking about without seeing the piano, but of the tone is really nice, the action is smooth and pleasant to the touch, the hammers don't have excessive/uneven wear and
and a lot of green stains on the strike point from oxidization, the piano might be a good buy.
Does any one else out there know if a Weber from that year is a US built or a Young Chang reproduction like the new Sohmers, Kohler and Campbell, etc. etc.? I'm not sure of Webers history and what year they sold out.
If it is a Young Chang, you may have at least a little room to negotiate if you still want it at all. Personally, I'm not a big Young Chang fan, especially compared to a US Weber.
I found a 2004 for a lot less.
If you like the piano and it performs well, buy it. If you have serious doubts, keep looking.
Search Webers on
www.pianomart.com and compare prices. Again, you can't see the piano. Just a fraction of extra knowledge.
Good luck.