[Richard]

I use McMaster Carr, because it is a day trip to the local HDT supply house.

Brass. SS is overkill.

As Simon said, most of the fittings will be 1/4 tubing by 1/4 npt, so straight and 90 in that size.
You will have a few 3/8 tubing by 1/4 npt, and a few 1/4 tubing by 3/8 npt. 1/2 tubing is used in a few places in the brake system.

I would also put in some 1/4, 3/8, and 1/2 DOT tubing into the spares box

Add some splices or connectors in 1/4, 3/8, and 1/2. They may save you if you get a hole in a line.

I love this combination for emergency repairs. The push on straight connectors and the plugs at the bottom of the page. https://www.mcmaster.com/products/fittin...for-air-8/. With 1/4, 3/8, and 1/2 you can quickly splice a line or use the connector and the plug to dead head the line. This is the only push on I use, and it’s just for temp repairs.

I would also add some extra ferrules and nuts.

And this last part is my OPINION. The fittings you get from McMaster will have sealant on the NPT threads. I use Rectorseal pipe dope. I do not recommend teflon tape on threaded fittings in our air systems. There is a possibility of shredding the tape, which produces little bits of teflon that will move in the air system and create problems in the various solenoid valves all over the coach. Yes you can be very careful not to get tape on the first two threads and avoid this. Yes you can also use way too much sealant and squeeze it into the system.

Loctite 440 is an excellent thread sealant. Pricey, and not avail at the local box stores, but it works and is easy to use.