To the OP:
It really sounds like you have the correct mindset about working with this horse. That's a great start! (Most people don't have a good mindset about these things.) It sounds like you realize it may take you 3 or 4 years to have a finished barrel racing product here.
I agree with barrelracinglvr that he doesn't have completely perfect conformation, but "he'll do".
The only red flag I would have is that old injury he appears to have on his left hind leg. Have you had him evaluated by an equine vet? It would be a good idea to check to make sure he is 100% sound on that leg, before you invest time and money into any training.
It also would be a good idea to have him seen by an equine dentist. It's always a good idea to make sure he doesn't have a tooth or mouth issue, before you expect him to respond properly to a bit.
If he clears the vet check and the dentist, then first things first: Get your horse broke, broke, broke. I like to be able to do these things with my horse before starting them on the pattern:
--walk, trot, and lope on a loose rein and relaxed
--stops easily and softly from any gait
--back up freely
--sidepass, move shoulders, move hindquarters, and move ribcage
--arc and counter-arc
--simple and flying lead changes
--direct rein and neck rein
--master "perfect circles"
Basically, you want to be able to move any part of your horse's body, at any time, at any gait. Softness and control! And making perfect circles is crucial to barrel training. I love Dena Kirkpatrick's method, where she teaches her horses to make a perfect circle on their own, while she only has one (inside) hand on the reins. This makes the transition to a barrel turn easy. She's got a lot of great videos on YouTube, but I would highly suggest buying her DVDs.
Here's one of her videos to get you started.
When your horse is just plain BROKE in general, then you can start patterning him. Spend lots of time at the walk and trot and make sure he does the pattern perfectly. Depending on how broke you get him at the start, will make a difference on how fast you can advance through the pattern.
And you don't always have to do the cloverleaf, you can do other random drills that still work on the basic fundamentals of barrels, without souring your horse on the pattern itself.
I personally like to haul my horses to a lot of different shows doing a lot of different things to get them used to going places and to keep a level head on their shoulders. This summer, I'll be taking my 7-yr-old to local shows where we will be doing halter/showmanship classes, western pleasure, reining, trail, and speed/gaming events. We'll also work cattle and go on trail rides. I'll be taking my coming 2-yr-old along to most shows too, as I can start doing showmanship/halter with him, and also get him used to sights and sounds of shows.
As far as what bit to use, I personally like an O-ring smooth mouth snaffle to start with. And I keep them in that as long as they work fine in it. There's no rules that say you have to change the bit for barrel racing. Use the bit that works for your horse. Out of the ones you posted, I personally would not use any of those for barrel racing. I like bits where the shanks will swivel on the sides, because that typically works better for the direct reining that happens in barrel racing. I also prefer a broken mouthpiece, but that will vary on the horse. Some work better in a solid mouthpiece. Gag bits are great because they give your horse a "warning" before the pressure engages, but horses sometimes can become "dull" in a gag bit, so you have to be careful of that.
And you already said that you are, but it is VERY important that you work with a trainer when you are getting your horse broke, and when you are training him on barrels. It is really, really hard to fix bad habits that can be formed when you don't know what you are doing. There is absolutely NO shame in working with a trainer! I myself have been riding horses since I was 2 yrs old and barrel racing since I was 4 yrs old (I'm now 26), and I'm even going to be doing some barrel racing lessons this summer. You are never, never too old or "too experienced" to learn something from someone else!
Keep us updated on your progress with him!