My personal opinion is that this old rule of thumb about the pencil through the mesh thing isn't very useful. I've never seen a dish less than 25 years old that doesn't pass the pencil test, but that doesn't necessarily mean the the dish will work well on Ku. On my first dish, which was C-band only, I tried to add Ku to it, but it was a very poor performer, so I tried to reduce the mesh size by covering the surface with fine metal screen material like you'd use on a window. This didn't help at all. I think the problem with old dishes is more related to the shape of the dish than to the mesh size. My current dish used to work fairly well on Ku, but now is terrible. The mesh size didn't change, but the shape did (because every winter I bang off the snow and ice with a broom, and it's now a bit warped and dented. I also have another 6' dish that used to work on Ku, but also got warped, and no longer does Ku. So I think that you really can't be certain that a C-band dish will work on Ku without trying it. My guess is that unless the dish has been mis-treated, that it will work OK, but if there are dents in the dish, or it seems to be warped at all that there may be problems. I've found that the most important thing for getting a dish to work on both C and Ku is to have the feed at the proper focal length.
Re the question about "what is the best lnbf", most people with BUDs don't use an LNBF, but instead use a feedhorn to which you can connect LNBs. However these usually have a mechanical polarotor to control polarity, and you'll need an old analog receiver to control that. There ARE lnbfs, which employ electrically switched polarity control that a FTA receiver can control. In general, these aren't as high quality as the LNBs used with the feedhorns, but they are much more convenient. I've never owned one of these C/Ku lnbfs, so I can't compare to the C/Ku feedhorns I've used.