With convertors you can only use a prime lens with a focal length of 135mm or greater. Any lens with focal below this will not work as the element sits too far back and will hit the convertor.
For zooms it is possible to use a convertor with a focal length as short as 70mm at the wide end of a zoom. Lenses such as 70-200's will work but a 24-70 will not.
In terms of what to get. I have a sigma 120-300 and it works great with my canon 1.4xII. I would reccomend canon's.
However with some of the third party convertors you can get AF
to work with lenses with apertures of F5.6 or more.
An example of this is the tamron pro 1.4x and the kenko 1.4x which both reportedly AF
with the 100-400 on a 1.6 crop body.
This is specualtion however, i have only used the canon myself and i will stick with them as they seem to be the best.
Any more questions, feel free to email me.
5D2/7D/1D2(soon to be a 1Dx) 17-40L/24-105L/70-200F2.8L/100-400L/24F1.4LII/50F1.2L/85F1.2LII