If I were to run a front derailleur, I would need to figure out the cable routing. Standard cable routing runs the cables under the down tube and then under the bottom bracket. This cable routing works with a front derailleur that gets pulled on from the bottom, hence the name 'bottom pull derailleur'.
Bottom pull derailleur
For a cyclocross bike, the cables sometimes run along the top tube, to keep them clean and out of the mud and gunk.
Two types of cable routing
That's the way this bike is set up. What you run into is the cables coming from the top, and needing a derailleur that is operated by pulling from the top and not the bottom, the aptly named 'top pull derailleur'.
Top pull derailleur
Often if you want to run a derailleur, you end up having the cable come down the seat tube, then around a small pulley, and then back up to the derailleur, converting top pull cable mounting to bottom pull. This is a nice graphic of cable routing in action:
The arrow is showing the cable routing.
I'm thinking I'm going to avoid the whole mess and run a single gear in the front and a casette in the back. This is popular for several reasons:
1. Simplicity: less going on, less to go wrong
2. Cheap: you don't have to get a derailleur, cable ect. You'll probably end up with a shifter in the front, because they often coem as a set, or you'll want them to match, and that can be a large part of the cost, so this is a small offset.
3. Reliability: the pulley is in a place where it'll take a beating from road gunk and the shifting will suffer accordingly.
So, with all that in mind, I think I am indeed going to run a single for now, and if I find myself missing gears, adding a derailleur after the fact is pretty easy. One consideration is a chain guide. The front derailleur acts as a guide for keeping the chain from flopping off. There are several things that can be done, adding a guide to the frame or chainring is the easiest, and I'm thinking something like this:
Some kind of cable guide
Whew, long post!