Nervous about committing to Single Speed

If you're building your own bike then you may well end up with a 42-tooth chainring from an old chainset. 42/17 is fine for spinning around town I find. I even sometimes go 42/18 if my knees aren't feeling too strong.

A quick quote from Sheldon Brown (here), which I think is worth considering:

You don't have to choose once and for all between fixed and freewheel, because the same bike can be both, if you use a reversible "flip-flop" hub.

Many cyclists interested in simplifying consider going to a singlespeed freewheel as a way to "test the waters" with the idea that if they turn out to like that, they might later convert to fixed gear.

This is generally the wrong way to approach it, in my opinion.

I STRONGLY recommend starting out with fixed gear. If it turns out to be a problem, you can easily convert to freewheel later if you want to...but my bet is you won't want to if you give fixed gear a good try (typically takes a couple of weeks of regular riding to get past the strangeness, but then it's quite addictive!)

Most folks who set up their bikes with a fixed/free flip-flop wind up using the fixed gear side pretty much all of the time. The freewheel option is mainly useful for when you have taken a longer-than-usual ride, and need to get home even though you're all tuckered out.