I just bought a threadless adapter so I so make use of the stem, handlebars, Shimano Sora brifters, and Tekro interrupter levers that were on the Felt F90 that I parted out.

I'm building up a Trek 1400 frame that I got off eBay, and although I already have a quill handlebar stem and handlebars for it, I wanted to try the Sora brifters, and the Tekro interrupter levers without removing them from the Felt handbars (different diameter from the SR handlebars and stem that I already had on the Trek.

By buying a chromed quill threadless adapter from the local Performance Bicycle Store for $14.99 (same price as online from China), I was able to drop the complete Felt F90 handlebar assembly onto the Trek, without having to wait for the adapter to be delivered from China.

Now let's look at your situation, and your questions.

The Performance threadless adapter comes with the adapter body, a quill wedge, a top cap, and the long bolt for the quill locking wedge. It works the same as a quill stem locking to the steering tube to hold it in place.

You do not need to buy a threadless headset, as you will still be using your conventional threaded headset. You do not need any spacers, because the threaded headset is still going to be holding the fork in place. Spacers are necessary on true threadless system to take up the extra fork steerer length so the headset can be secured. Since you are still using a quill system, the spacers are not necessary.

The threadless adapter only has around 1" of vertical adjustment. If you need more rise than that, you need to look at threadless handlebar stems with a different angle or rise, OR buy a threadless extension adapter which will attach to the top of the threadless adapter. Adding height to the handbars is a PITA with threadless headset systems. It's not like the ease of buying an extra long/tall quill stem.

The cap that comes with the threadless adapter is a smaller diameter than is used on true threadless stems. The cap is trapped because the quill bolt can't be removed. Removing the quill bolt and the cap would drop the lock wedge. The threadless adapter cap is not used to set the preload/tension on the fork and headset. The larger diameter of the cap on a true threadless system is to keep the handlebar stem from sliding over the cap before the stem bolts are tightened. The adapter cap is a smaller diameter so you can slide the threadless handlebar stem onto the threadless adapter without removing the threadless adapter and sliding the handlebar stem on from the bottom of the threadless adapter.

So to go to a threadless handlebar stem on your threaded headset bicycle. All you need is a threadless adapter (as long as the limited rise of about 1" is okay), a threadless handlebar stem with a clamp diameter that is compatible with your existing handlebars, and some handlebar wrap to rewrap your handlebars if you need to unwrap them to move the handlebars to the new threadless handlebar stem.