Yep , me to , I am now trying to download it for the 6th time today , and I have not even started on Horizons yet.
Because , I had the 32bit version before , so as far as I understand , you cannot download the horizons update until you have the 64bit installed , and if I am wrong , I don`t care , as the problems with update compatibility I have been having , means that this is probably the way to go , for me at least.
You only need to download the horizons update. You do not need to download 1.5 64bit version first.