I don't think you can come up with a single rating for each product, as different people weight the features differently. For example, stylus input is quite important to me, but to others, much less so. But your idea of a spreadsheet where a potential user could weight several factors and then get ratings of the different units would be good. You've got a nice list of requirements to start with. I'd add the OS on the unit -- I strongly prefer Linux-based models, myself. There are also folks who want to be able to interface with the unit using Linux, rather than a PC or Mac. That could be another factor.
Tiki has spreadsheets built-in. I have a couple of Tiki installations and could possibly donate space, if that would be helpful. Tiki also has polls and surveys available, so you could let users rate the reliability or ergonomics of various units, trusting that people won't rate units they don't own.