I currently commute about a mile to work each day in upstate New York. For the past year, my ride has been a 2012 Specialized Tricross with a 3x8 setup, which has treated me very well in this snowy and hilly area. However, my ideal commuter would be a cromoly frame with only a rear derailleur, perhaps 1x8 or 1x9 with a 11-32 casssette and trigger shifters.

I just came across a brand new single speed State Bicycle Co. Brigadier frame (link to specs) on CL for $200, which is half what the bike would normally cost. It's in pristine condition, so I'm thinking about using this as the base for a new commuter.

My question is: What kind of issues might I run into with this conversion? What might this cost (I'm looking to watch my money, but I don't want to skimp on quality)? Any recommendations for reasonably-priced and reliable parts?

Don't go this route. Start with a decent used geared bike. The rear dropout spacing won't work for geared bike wheels. There are no stops for cable and housing, there is no derailleur hanger. You'll need a new rear wheel, derailleur, and shifting system which will probably cost more than that bike... or a decent used geared bike.

Starting with a fg/ss bike when your goal is a geared bike is a Bad Idea.

You'll need some kind of derailer hanger like this . I think you can get away with zip-tying a full run of housing (in the absence of cable stops) from the derailer to the shifter. You can use a clamp like this to mount a downtube shifter.

The website doesn't give the rear axle spacing. I don't know if wheel would need to be redished to fit a 120mm axle and a cassette.

I don't know how you would mount brakes on that. You could drill out the fork for a front brake? Not sure if the frame has a rear brake bridge that could also be drilled out. Again, in the absence of cable stops, you'd need a full run of cable from the brakes to the levers.

Thanks mconlonx!

I'll now add this idea to the bin marked BAD IDEAS. There's a whole bunch of ideas in there, but at least this one didn't cost me a cent.

Guess I'll take a second look at that mid-2000s Specialized Sirrus Elite or that vintage Pista I saw on CL...

Just use you current ride? Just take off the front der and other chainrings. Put on what you need for the front, 42T? Get a rear cassette that you want.

If you want gears, buy a Shimano Alfine hub and the controls and you have a geared bike.

Derailleurs and a cassette won't work well with a SS conversion and isn't worth it.

Sram and S-A offer hybrid IGH cassette combination rear hubs, there is an internal 3 speed so you can use only 1 chainring.
requires 135 wide rear spread
narrow fixies 120 are only practical to put a 3 speed hub in them , though there is this kit : Six Fyx Conversion Kit | Fyxation

I got the fixie bug a couple of years back and have a beautiful old Raleigh International that I made up to commute. Got tired of the fixed thing and recently went to Campy 8 in the back. Had the rear stuff lying around, LBS set me up with a FD and Sugino Crankset (47/36) that they had in their parts been. Friction downtube shifters. Rides great!! Much happier commuting this way.

The only way that this was an easy swap for me is that the frame was already set up for the derailleurs, etc.

+1

THat is the right answer - for $250.

You could also put on a slumpf drive on the crank, but that is only 2 speed and about the same cost.
Schlumpf Innovations Gearing Systems - Speed Drive, Mountain Drive, High Speed Drive