Depends on your needs, what type of hight do you want to hit and volume of use will tell you the size of tank, pump, engine, and hoze to be used, then the gun tip is soemthing that gets changed as distance changes. Adjuvents and surfactants will help some too.
you loose some efficiency with spray hoze length too.
You want to be able to maintain an unfractured column of liquid up to the target highth. Pressure alone wont do it, volume and column width are factors also.
So if you want to spray 60ft trees you need something like a 12hp engine, 30gpm pump, 200 gal tank and 3/4 inch hoze. If all you want to do is shrubs and 20 ft trees then a small centrifical pump with a 5hp enging and 1/2 inch hose and a pistol type gun.
Small trees is where most of the money is in anyways. Big trees are a PITA because of the usage and drift issues.