The HT2411 is a stand alone hand held traditional hedge trimmer.
The HTA2000 is the hedge trimmer attachment for the power head unit that can be used with the EP7500 extension pole. And yes you would need a PH1400 power head to power the hedge trimmer attachment.
Look at the kits and options for the best price. i.e. you can buy the edger kit ME0800 for less than the power head alone. Also determine what battery you need if you do not already have Ego battery. Kits can also save money there.