How tall do you want the hedge to be? I cut my buddlejas down to knee-height every March ... while it doesn’t take too long to get back into vigorous growth, it’s not what you would describe as a ‘regular hedge’.
It depends.I have a buddleia "masquerade" which can get to 1.5 metres high and ide but another non variegated buddleia which gets to 3m high and wide without any encouragement. The Buzz series is dwarf and grows less tall and wide.I suggest you select the varieties you like best then check here for eventual height and width - https://www.rhs.org.uk/plants/search-results-beta?form-mode=true&query=Buddleja davidii Your planting distance will depend on eventual width of the plants chosen.
Yes. If all the plants grow 2.5 metres wide they need to be at least 2m apart so you get some overlap but not congestion.