Dear Justin,
The Calendar is not an issue with the
Unabbreviated Horologion (or any other Horologion for that matter). Any dates given (for troparia, kontakia, etc.) will be the Church Calendar date. Example: Christmas will say '25 December' so those on the Gregorian (or Revised Julian

) Calendar will be celebrating it on 12 December according to the Julian Calendar, while those on the Julian Calendar will be celebrating the Feast on the
real 25 December (which many people foolishly call January 7th.)

An Horologion will not be enough to celebrate the services if you plan on doing Vespers and/or Matins. For these services the Octoechos, Menaion, Triodion, and Pentecostarion will be necessary, though you would be able to get along with an Anthologion, if one were available. The only Anthologion I know of in English is Nassar's
Divine Prayers and Services available from the Antiochian Orthodox Christian Archdiocese of North America, though there may be a Greek Catholic Anthologion of which I am not aware. Alternatively, there are internet sites on which these texts are available and you could download what you need for each service.
Fr David Straut