I originally posted those instructions for peace of mind for our users and because the factory require the information before getting R&D involved. In the UK we have had only two instances of corrupt checksums in the past year or so (one from a member of this forum and another from a second hand Audya with a very checkered history). What the test enables us to do is eliminate faults and identify user error.
Here is what happens:
1) Customer contacts us (often frustrated and slightly angry) to complain of fault on his/her Audya - especially after reading some of the negative tosh posted on this forum at times!
2) We ask them to run the tests which normally come back clear (except KetA02 which we expect)
3) That is usually enough to calm them down & the customer becomes open minded to the idea that the the fault may be something they are doing wrong and we can then talk them through the solution.
This procedure has worked really well over here. If you search the forums I doubt you will find many...if any instances of corrupt flash
However...there are some things that can cause checksum errors.
Reloading old USB updates or downloading incorrect OS versions (there are warnings on the website) definitely cause problems and should be avoided. Used Audya's, if they have been looked after correctly should be ok, but there is no way of knowing for sure. Simply ask the seller to run the tests for you. if they are a genuine seller they will do it. If they refuse I would be very suspicious of purchasing from them.
Saying that, I understand your problem and can see why this could be a potential issue when choosing a keyboard. You just need to make the choice based on what you will be most comfortable with. Personally, unless your country has very good support I would avoid a used Audya. In the UK we offer a service to those who buy pre-owned Audya's (there is a small charge) where we can arrange collection, re-installation of factory content followed by all updates and redeliver back.
I think Ketron are the easiest arranger to use for partially sighted and blind and we have a number of such customers in the UK.