I thought I might be able to be more help, I looked at Kurzweil's site and downloaded their manual for the Mark 12, but I didn't find a list of the different sounds. I have four Kurzweils, each of them has different sounds in different places (K2000VP, PC2, K2661, and SP88X)
I also play in the church praise band as the second keyboardist. Our primary keyboardist uses a Kurzweil PC88 set to one of the electric piano sounds. I started with the church playing electric bass, and I use some of the electric bass sounds for a lot of the fast music (I also have an eMu MIDI keyboard running a computer that is running a soft synth which gives me Hammond B3 with Leslie sounds). On the slow sounds, I use one of the Grand Piano, Dymo Electric Piano, Slow String Cello, Cathedral Choir, Wendy's Flute type sounds depending on what is needed to add a fillin without getting too busy sounding.
I would encourage you to get hold of the full manual (it can be downloaded from
www.kurzweilmusicsystems.com under the Support section) and try different sounds. I did read that it contains about 300 sounds. My 2661 has over 600 sounds, and I only use about 20 of them with any regularity. I leave the 2000VP at the church so I don't have to haul stuff back and forth, it has 200 sounds plus I can add others from a hard drive that I have attached.
On your other questions about good chords and runs - I'm in the same boat, I don't yet have a lot of keyboard experience and am still learning. Topics on this site have been helpful to me, and I probably will order one of the DVD courses, I seem to learn better when I can see and hear something demonstrated than just reading and trying it on the keyboard. There are a lot of VERY good musicians on this board (and many precious saints of God), and some of them will likely chime in with advice. Some may even have experience with the particular model Kurzweil that you have at your church.
Blessings,
Jim