From top to bottom it is system, department and channel/TG that you select to hold on to. I would say that it is channel/TG that get used the most.
The range doesn't slow down scan very much. If there is no carrier on a channel, due to it being out of range, it doesn't stop on it to check if its valid and tries to decode it. The hold time for a trunked system only come into play if there is a carrier detected. But if you only have an indoor antenna I would say that 50 is a bit optimistic.
/Ubbe