Up until about 5 years ago I had a couple hydro walk behinds that were 1998-2000 models. I thought they were a great bang for your buck. The Z riders were decent as well. The one thing I didn't like on the Z riders was the electric deck lift...they didn't last long enough to make it worth having. It had a button for raising the deck and lowering it back down. If the employee didn't hear that the deck was all the way up and kept the button held down it could ruin the electric lift arm rather quickly.
At this point with the mowers being as old as they are it really comes down to the maintenance practices of the previous owner. It doesn't matter how good of a mower they were then if they failed at keeping them maintained.
Best of luck.