What should I look for in an IBL?

First define your goals. Are you growing for yourself or for commercial production? Indoors or out? Mostly sativa or mostly indica? Keep only the plants that have the qualities you want, and mercilessly kill the rest.

Or Select plants that don't fall over; if you have to prop them up with toothpicks, you might as well cull them. Keep the ones that are free from abnormalities and hermaphroditism. Hopefully you've got enough many seeds to be ruthless. Keep the ones that show better resistance to disease and pests. Even though the progeny performance is more important than the individual, there is a positive relationship between the overall vigour of an IBL and the yield of its hybrids.

If they produce vigourous plants they are more likely to pass these traits on. Then employ a breeding strategy towards homozygosity, such as recurrent selection, selfing cubing.