I'm replacing the fork on my bike, since it's been bent in a collision with a car.

While I'm at it, I was thinking about switching to Threadless.
Since my quill stem is fixed at 45°, and I've always wanted it to be lower.

Which of these options seem best to you?
In cost, convenience, performance...

1. Buy: threadless fork + threadless headset + threadless stem.

2. Buy: another threaded fork + threadless stem adaptor + threadless stem.

Currently the fork, headset, and quill stem on my bike are the 1" size.
I'm not sure if I need to stick to the 1" size, or if the headtube can accommodate switching everything to the 1 1/8" size?

Thank You!!

First, if your headtube was made for a 1" headset and foirk, you must stick with 1". Other than having a frame builder remove and replace the headtube with a 1-1/8" one, there is no choice and that option would be absurdly expensive.

As long as you have to replace the fork, you might as well go threadless all the way so option 1 is my recommendation. You will have to buy and install a threadless headset but, overall, the new set up will be lighter and 1" threadless headsets are not that expensive (at least not if you avoid Chris King or similar).

Although some forum members might disagree (see numerous discussions on these threaded unthreaded headset issues in this forum), it is not absolutely necessary to find a threadless headset. You can make the threaded headset work for an unthreaded steerer: the most significant modification is to file out the threads of the upper lock nut so it fits over the new steerer tube (some washers (separators) might need a 'nose' removal too).
As proof of concept see https://www.petracycles.co.uk/headset-adapter-inch-silver-p-10505.html , a set which is not really needed, since filing of the head nut will do. Just the star-nut is needed, which every bike shop (hopefully) has.
Good luck!

I'd go threadless. I can't see installing something new knowing you'll need to use an adapter if there is a reasonable option.

You must file out the threads of the top bearing race, not the locknut which is unnecessary for a threadless headset. However, for decent bearing life and proper steering, these threads must be removed uniformly and have a smooth, close tolerance slip fit over the threadless steerer. If they are off center or a sloppy fit there will be steering problems and fork play. Remember, there will be no split cone centering ring in this arrangement. This is a job for a lathe and an operator who knows how to use one, not a guy with a file.

Pro mechanics used to do this on Dura Ace headsets back when Shimano sponsored teams had to use all-Shimano components, even though they never made threadless headsets. It's a stop-gap at best and a kludge at worst. Get the correct purpose-built headset.

Treat yourself. You are replacing the fork anyway. Why not go for the threadless headset & stem with the new fork? Heck, throw a set of 31.8 ergobars & some new bartape on there.