I have paddled a fair bit of whitewater up to Class IV, mostly in open boats but a good bit in kayaks. The main differences you will encounter in switching from a kayak to an open boat are independent of the exact boat model and are as follows.
1. You will not have a strong brace on one side.
2. The strokes taken on the two sides of the boat are not symmetrical in whitewater.
3. You will take on water in big waves and usually when going over sizable drops.
The first two factors results in nearly all open boaters having a strong side and a weak side and all but the real experts tend to favor their strong side as much as possible. Although a few open boaters are ambidextrous and can paddle equally effectively on both sides, they are few in number IME.
The fact that an open boat as opposed to a decked boat can quickly take on a very significant weight of water will tend to limit what you can safely run. Some expert open boaters have such a solid roll that they can paddle Class V water, but they are again few in number. And even if you completely pack the open boat with as much flotation as you possibly can, the weight of water that it can take on around your body is still enough to make the boat sluggish and difficult to control. Catching small eddies in a wet boat with a lot of momentum can be very challenging. And even if you develop a solid roll in an open boat, you will still come up wet.
The fact that the strokes are very asymmetrical on the two sides of the boat might pose the greatest challenge initially. Many kayakers going to a C1 or open boat find it difficult to make the boat go straight. That is particularly so for highly rockered hulls, and especially so for today's very short, plastic open boats.
In a kayak you have a relatively symmetrical forward stroke on both sides so the yaw effect of the forward stroke tends to be balanced out when taking normal forward strokes on opposite sides. In a canoe you will need to either combine forward strokes with cross-forward strokes taken on the two sides alternately or in some ratio, or you will need to repeatedly switch paddling sides like a marathon canoe racer, or you will need to combine a forward stroke with some sort of correction stroke added to the stroke on the same side.
Repeatedly switching sides is discouraged by most whitewater open boat instructors. One reason is that there often simply isn't time. Another is that you may lose your grip on the paddle during the switch. Using a forward stroke/correction stroke combo works just fine for flat water canoeing and lesser whitewater but it won't cut it for significant whitewater, especially in today's modern whitewater hulls. In those boats a single forward stroke will turn the canoe 90 degrees or more. Trying to use a forward stroke/correction stroke combo will kill so much forward momentum and slow your stroke cadence so much that you won't be able to cross strong eddy lines when exiting eddies, or climb onto surf waves. To get those little short boats moving from a dead stop in the direction you want will require you to use forward and cross-forward strokes on alternate sides of the boat. Since cross strokes are something you don't usually need to use in a kayak you might not have developed an effective cross-forward stroke. Cross strokes can be ergonomically challenging for most.
To keep those short boats going where you want without wasting too much energy, modern whitewater open boats use "hull carving" to a great extent to paddle in arcs of varying radius. Kayakers use that technique too, of course, but it is perhaps not quite so essential as in a modern whitewater canoe.
I would not necessarily turn your nose up at a longer "old school" whitewater canoe. If you live in the southeast US there should be tons of whitewater canoes going unused because their owners have switched to a short plastic boat. Starting out in that type of boat can make the learning process a little easier because they do not spin in place quite so readily as a more modern canoe. That would allow you to get more comfortable controlling the boat in the kneeling posture and allow you to work on your cross strokes in a somewhat more user-friendly hull. Those older boats can be paddled using more of a "point and shoot" technique on a lot of less-technical whitewater.
As for a specific make and model of canoe, I don't think anyone can offer you a useful suggestion as there are just too many unknown variables and individual preferences also come into play to a huge extent.
I know that the Silver Birch Covert has become very popular in recent years and I have a friend who owns one, but I have never paddled one.
I have a Blackfly Octane 85, which is a somewhat edgier version of the extremely popular Blackfly Option and I like it and prefer it to the Option, but to be honest I usually prefer paddling what you would call and old school design these days. Another very popular hull has been the Esquif L'Edge. There are also the partially decked boats with very low forward and rear decks like the original Spanish Fly, the CU Fly, the Dagger Quake and Aftershock, and the Big Dog Force. All of the above have their fans.
If you have the time and inclination I would try to attend at least a portion of the Ain't Louie Fest (ALF) which should be March 10th to March 18th this year (but I would confirm that if you plan to attend). It usually kicks off with an "over the falls" race on the Tellico Ledges which is from the bridge put-in to just past Baby Falls. Other runs are planned on a daily basis depending on participant desires and water levels. If you go you will likely see just about every variety of whitewater canoe made, old and new. If you go and you are interested in Blackfly canoes try to look up a fellow by the name of Alex Vargas. Alex has been a representative for Blackfly and usually has some of Jeremy Laucks' boats available to try out, or at least he did as of a few years ago. You can find Alex on facebook. You can also check out the FB group titled "GDI".