Rather than convert the Holy Apostles itself into a mosque, Mehmed decided to demolish it and build a mosque of comparable magnificence on the site. The result was the Fatih Cami (Mosque of the Conqueror), which still occupies the site and houses Mehmed's tomb. There is a limited visual record of the Holy Apostles, but St Mark's Basilica in Venice is partly modelled on it. The 12th century writer Nicholas Mesarites has left a description of the church (see external link), of which only parts survive.
* [http://homepage.mac.com/paulstephenson/trans/mesarites.html Nicholas Mesarites, Ekphrasis on the Church of the Holy Apostles]