Check with the low vision support organisation for the market your product is aimed at - for me in the UK that would be something like the RNIB - They should be able to tell you what specialist keyboard shortcuts their users are familiar with. You could call that a benchmarking exercise.
There are a couple of reasons why letters are generally used more than numbers. The first is that we can more readily form an association with the intended action; CTRL+P for instance is the short cut to Print under Windows OS just as CTRL+S is Save and CTRL+C is Copy. (In case you were wondering, 'Cut' and 'Paste' are more visual: 'X' is the closest thing to a pair of scissors and 'V' looks a lot like a downward facing arrow to signify placing something down after carrying it... and it makes it possible to have these related functions close together on the keyboard.)
The second reason is that the standard resting position for someone using a full keyboard is generally over the letters with their index fingers on the "Home" keys ('F' and 'J'). For people with low vision this is especially true and, on most keyboards, the home keys have a little raised bump to help locate them without looking. This may be the reason for choosing 'J' and 'K' for 'Up' and 'Down' or 'Previous' and 'Next'.
When choosing your shortcuts you should avoid using combinations that are more commonly associated with other popular functions (such as CRTL+P, CTRL+S, CTRL+C, etc.) and gravitate towards patterns that your users may already be familiar with. This way you reduce the learning curve required for your product and reduce frustration in the user.
Scott Hanselman wrote an article about common web keyboard shortcuts that explores the shortcuts used by many commonly visited websites where you can see some common patterns.