If you are considering a viewer other than the suggested models, you’ll want to make sure that it has both achromatic lenses and geared focus/interocular adjustment. The Kodak Kodaslide viewers for example have good lenses but friction-based adjustment, which reportedly can get sticky or locked-up in some examples, and are best avoided.
If you are shooting medium format, finding a good viewer can present a real problem. From what I understand, the SaturnSlide viewer and late-model viewers made by 3D World were among the best, but I’ve yet to see either of them on eBay. Some other viewers are available from time to time, but they all either have some sort of design issue or lack features you would want. You can either settle for one of those, modify them to address their deficiencies, or attempt to build your own.