Chris,
I have an Optivisor that I have had for going more than 10 years. I started with the number 3 (dioptre) lens and as my eyes and reading glass prescription have changed I changed to the Optivisor number 5 lens. In retrospect I should have probably started with the number 5 as it's approximately 8 inch working distance is right around where I am most comfortable - the number 3 dioptre has a working range around 14 inches.
The lens number you will have to choose based on your working comfort range. On the link that Joe provided (and provided again for convenience;
http://secure.transtronics.com/Optivisor.html ) towards the bottom of the page they list the different lenses and working distances.
One option for the Optivisor is called he Optiloupe (you will also find on the same web page). I have one and don't know how I ever got around with out it. It replaces one of the lens retaining screws and adds a flip down loupe that adds 2-1/2 times power additional magnification. Choose the side with your best or dominate eye.
And do get the one with the real glass lens not the cheaper plastic lens.
I use mine all the time, one of the first things I grab when I go into the shop to do anything, even if I really don't need; it's on my head and ready to use when I need it.
cheers, Graham in Ottawa Canda