Before you mount to he existing pole make sure it is plumb. This is not even important with a fixed dish install like Dish Network or DirecTV but to help you with ease of install of a
motorized system the time you spend making sire that pole is plumb will be well worth it. Also you want minimal flex in the pole. The motorized system is quite a bit heavier than your old dish you had on it.
I use 1 1/4" black pipe in 10' sections for installs but that type of pole has quite a bit of flex so I fill it with concrete. Others use a galvanized pole.