I was in a similar position some years ago and decided to take up the challenge - I've now been playing for 18 years, not brilliantly but well enough, and really enjoy it. Take heart that is is much easier than on the piano to sound convincing, even in the simplest music, thanks to the organ's sustain and potentially richer sound. You can lead the hymns quite satisfactorily playing only the treble and bass lines (though if you have a SATB choir they may want more), and there is lots of straightforward but attractive manuals-only music for voluntaries.
The 5-vol Cloister Album series has already been mentioned. I also use the following:
Manual Miscellany for Organ (2 vols, ed. CH Trevor)
Seasonal Choral Preludes for Manuals Only (2 vols, ed. CH Trevor)
Old English Organ Music for Manuals (6 vols, ed Trevor)
The Essential Organist for Manuals: 60 pieces for weddings, funerals and everyday use, (Kevin Mayhew)
For some easy Victorian schmalz try voluntaries by Caleb Simper, all written on 2 staves with optional pedal - it's not great music but a lot of fun!
I was able to buy all this music second hand, either from Alba Musick in Glasgow or UsedOrganMusic
When I first started playing for services I was terrified, unable to sleep the night before and physically shaking at the organ. However I had lots of support from the minister and congregation, and I also discovered that most people don't notice mistakes, even really crashing ones, so long as you keep going without hesitation. I'm relatively blase about it now.
Go for it!