what he means by horns are the part the parabolic dish bounces the signal into. you know that little (normally cylindrical) piece that is out on "arms". on normal dtv dishes the arm comes out on an angle with said peice attached and is referred to by them as a horn. by requiring three horns he means you need three of those receiver parts. i'm guessing that your big dish doesnt support this (but check with your local directv agent to discuss this further!, dont just take our word on it)
i'm not sure what he means about the UHF antenna, you'd have to check with directv to ask.
i've heard that the dish has to be aimed precisely but you'd have to check with directv (or check your signal yourself) to verify tolerances.
in any case, call directv to ask for some input as they should know what HD requires. if the person on the phone doesnt know, talk to their tech/service department.