To start with, as fietsbob mentions you need something to tension the chain. I highly recommend an eccentric bottom bracket for this. I've got the Problem Solvers PF46 EBB on my 2013 Jake the Snake, and it has worked flawlessly. The same basic design is also available from Beer Components and Wheels Manufacturing. Wheels Mfg and Beer Components both make a BB30 variety of their EBB. I believe for the Crockett you'd need a BB30 EBB but with longer bolts. I'd try reaching out to one of these companies to see if they can help you out with the right length bolts.

For crankset, you can use any CX crank (with a long enough spindle) and just get drop the inner ring (which also means shorter chainring bolts). If you use an EBB such as I describe above you'll need a 24mm (or 24/22 if you prefer SRAM) crank, but I think the weird bottom bracket on the Crockett would lead you in that direction anyway. The Shimano CX50 is a decent crank that you can get fairly cheap. I would recommend the Ultegra 6800, but its non-standard bolt pattern would limit your chainring options and for a singlespeed build that's a bad thing.

I've been happy with Surly cogs and chainrings. They're a bit heavy but durable and at this scale "a bit heavy" might mean an extra 50 grams. Assuming you have a reliable solution to chain tension and you pay attention to your chainline you don't need anything like narrow-wide chainrings because the chain is kept on just by being the right size.

For disc brakes, I definitely think TRP Hylex are the way to go. Hydraulics are a big step up from even the best mechanical options.

Your plan for wheels sounds good. I bought a bunch of spacers from Purely Custom. I don't remember the sizes I got. The key thing is to get a couple of wide ones and then a half dozen or so thin ones in various sizes so you can fine tune the chainline.

One caveat for the wheels is that if your wheels have an aluminum freehub body (and I think the Grails do) the single cog will bite into it more than full cassette does. The Surly cogs are relatively wide, which helps a little with this problem. Depending on how much power you put out, you should be prepared to have to replace your freehub at some point. This isn't an issue with steel freehub bodies.

Gearing depends on the course, how strong you are and whether you prefer to spin or mash. 39-16 or so is probably a good starting point. I use 39-18, but that's pretty low and slows me down. That's OK for me because I'm pretty slow anyway and just racing singlespeed for kicks. If you want to do well, you'll need something higher. Bigger cogs are slightly more efficient and considerably more durable, so resist the temptation to use tiny cogs and chainrings. You can get the same gear with a bigger combination.

I generally use a SRAM 9-speed chain. If you get a second master link you can set it up so that you can keep a couple of different short sections to attach to give you more options for changing chain length as you change gears. The range of gears you're likely to use for racing might be small enough to manage with just changes in the EBB, but if you ever ride the bike on pavement you'll want a much higher gear that will require a longer chain.