It pays to be conservative with cutting the steerer tube. If you are ordering a new frame, ensure that whomever is selling it to you does NOT cut the steerer tube at all. LBS are terribly guilty of cutting steerer tubes on all bikes way below what is practical for many riders.
If the steerer tube is left long, you can fiddle with the spacers and get to a ballpark height comparable with your current quill set-up. Yes, there are stems in various lengths and angles, but it's much better to settle on one to start with and stick with it. Check the length and angle to the quill stem you have, and transfer those dimension over to the threadless one.
The issue with leaving the steerer tube uncut, of course, is that you will have to add spacers above the stem. You'll also have to insert either the star nut or expander plug so you can tighten down the cap and get enough preload on the headset bearings. But don't worry.
Once you have ridden the bike a little and either moved the stem up or down to suit you, or settled on the right height, you are then in a position to have the steerer tube cut. If there is a star nut in there, it will either need to be left in place until after the cut, then knocked through the cut-off bit, then reinserted in the steerer; or if it is right where the cut is to be made, knock it down far enough to make the cut.
It's always a good idea to set the height based on the required number of spacers below the stem, the stem itself, and 1/8th inch/3mm spacer on top. Once you figure this height, cut the steerer tube according to the instructions, ensuring it is slightly short to account for the depth of the cap.
The additional spacer on top of the stem enables a little bit of further fine-tuning if needed, but on a CF steerer tube, it also helps prevent the stem from causing cracks around the top which then can work their way down the tube, and thus weakening it.
Hope that helps.