Not here to restart the timeless SS vs. Geared debate, but I'm facing a bit of an internal dilemma and would appreciate your $0.02.

My beloved Frankenbike was stolen a month ago. Miraculously, I just found it outside a church two blocks away. Unfortunately, the thief did a number on it (swapping out parts + terrible spray paint job), and now I have to rebuilt it.

Should I:
a) Rebuild as a geared bike? It would need new front wheel, both brakes, cassette & chain.
b) Rebuild as a single speed? It would need new front wheel, new free wheel setup, both brakes, chain, and anything else I'm forgetting.

I live in Portland, OR. This is my beater bike for commuting around town. I know the pros and cons of each style from a riding standpoint; I'm more curious in your perspective around initial rebuild costs, ongoing maintenance costs, and reversability (i.e. if i end up not liking SS, how tough and costly would it be to switch back, and vice versa). Thanks!

Come on mate, make your own decision. It's not up to us to tell you whether to ride geared or SS. You're the one the bike and you're the one who knows your roads. As for costs, geared is always going to be more expensive because you need more bits, but if it makes the bike you want, the price is pretty much immaterial.

You should leave the bike behind the church and buy a new Kilo TT

Everyone is all about the damn Kill TT's haha.

But you should buy one..

One of the best bike purchases I ever made!

Purely from a cost standpoint and all else being equal, singlespeed is much cheaper than geared over the long term. You can get away with replacing chains less often, and you can often find SS chains for cheaper if you're so inclined. But there are cheap ways to go either way as far as initial setup cost goes, and maintenance on a low-to-mid-range geared drivetrain isn't prohibitively expensive.

If you have (or can find for cheap) a rear wheel that's already got threading for a freewheel (instead of a splined freehub body) and you have horizontal dropouts, all you need to make it a singlespeed is a freewheel and a chain, both of which can be very cheap. If you only have a wheel with a freehub, you can still get a spacer kit to make it a singlespeed, but you might spend a bit more. But cassettes can be pretty cheap too, and so can chains.

The real question is, what condition are the derailleurs in, and the derailleur hanger? If the person took them, swapped them, or bent/broke/busted them, the cost to rebuild it with gears goes up pretty quickly.

But yeah, just build it the way you want it. SS is probably cheaper, but probably not by enough to outweigh your actual preference.

Remember the last guy who got convinced to buy a Kilo TT and then started complaining about the toe overlap? Me neither.