A marriage ceremony which takes place in Austria is only legally binding if effectively executed by a registrar (the principle of so-called "compulsory civil marriage"). A purely church marriage is insufficient, unlike for example in Spain or Italy.
The desire to bring an "own" registrar or to have the ceremony to be guided by a friend can be accomodated for by a "Doppelconférence": the chosen friend or relative may take care of the exchange of rings and hand over the marriage certificate while the registrar actually responsible for the ceremony may deal with the set questions and answers, the notarization of the declarations and the registration of marriage in the central civil registers.For marriage abroad the general rule applies: if the form of the marriage follows the law of the country in which it was concluded, the marriage is also valid in Austria. Therefore, for example, the sacrament of marriage conducted in Spain or Italy is in full force and effect in Austria.
This said, within the context of marriage in Austria there are sometimes legal issues which may arise should you or your (potential) spouse have already married and divorced abroad. The registrar then has to assess whether the divorce was effective abroad - otherwise you or your partner in Austria would have married twice. Should you wish do discuss the details of the recognition of foreign divorces I will be happy to help.