Without knowing your skills it's tough to make recommendations, but I'd start by suggesting you take a look at the hymns in the Advent section of the Episcopal hymnal and select tunes from there. I know one of my golden oldie favorites is Bach's "Wachet Auf" (Sleeper's Awake). It's tricky though. Also, Bach's "Nun Komm der Heiden Heiland" (Savior of the Nations, Come) from the Orgelbuchlein is another beautiful standard. If not sung, start off playing the chorale just on a 4' flute, then play the setting. Also, both of these are quieter pieces, well suited for Evensong. There are bazillion arrangements and variants on "Veni Emmanuel" (O Come, O Come Emmanuel), at all levels, that would work. I'd suggest seeing if they know the hymns to be sung and use one of those tunes for your prelude. It will tie into the service nicely. Also, for the postlude, I like to keep it quiet and simple. Folks will remember you more for the "AHHH" rather than the loud. Also, Advent is a more pentitential season, so quieter is more appropriate, at least in liturgical services.
I just looked at your web site... some lovely works there, guy! Why not adapt your piece "Snowfall" and use that for a prelude if you want a non hymn tune selection? Use either string or flute celestes at the beginning, then move to a closed swell or choir division. If there's midi w/ bells, bell strings or such, by all means go for it. Toot your own horn and play your own music!
And as a side note, God Rest Ye Merry Gentlemen is really more a Christmas hymn than Advent if that's of importance to you.
Good luck and have fun!!! :)
Holtkamp/Parkey pipe organ
Galanti Praeludium home instrument