I shoot ICORE and Defensive Pistol. A 4" 686 with either a Pinned front sight, or the quick change front sight would not be a bad way to get started. With a 6 shooter you can use either Safariland Comp III speed loaders, or Jet Loaders. With a 7 shot 686 you are limited to either doing a Moon Clip Conversion, or using HKS or 5 Star speed loaders.
You will want a Fiber Optic front sight.
I use Comp III's with my 686. With a K frame I find Jet Loaders to have an advantage.
I also have a 4" 610 shooting moon clipped 40 S&W, as well as a 625 shooting moon cliped 45acp.
We have several shooters using moon clipped 686 Six Shooters, and GP100's. Surprisingly Not for Speed. For ease of picking up brass. In my local bunch of shooters, I have no one running a 7 or 8 shot revolver on a regular basis. I have one guy who shows up now and again with a 686 Plus running HKS speed loaders.
In ICORE you are in Limited 6 with a Moon Clipped 6 shooter, and Classic with a 6 shot Speed Loader Revolver. So you are not shooting against the 7 and 8 shot revolvers.
In IDPA again you are limited to 6 shooters.
I will probably be stones, but a GP100 is not a really bad option either. The Ruger with adjustable sights comes with a Quick Change Front Sight. So no problem at all installing a Fiber Optic Front Sight. You will need to sort through 686's to get a Pinned Front Sight gun so you can change the front sight.
We are not the norm, but at our Club we have more GP100's than 686 shooting ICORE. Most are either 3" Willy Claps, or 4" Match Champions. I go back and forth between my 686-5 with 4" barrel, and my 6 Inch GP100. My little brother shoots a 6" GP100 as well as a 3" Willy Clapp when shooting 38 specials. Most on here will tell you how much better the S&W trigger is over a GP100. I have 2 each 686's that are Power Custom Combats, as well as 2 each GP100 with trigger jobs. I do not find enough difference in the triggers to worry about. All can be made perfectly acceptable. My brothers Willy Clapp GP100 is a Power Custom Combat also. No one is going to complain about the trigger on it.
My advice is get a 6 shot 686 or GP100.
Watch Midway for Safariland Comp III speed loaders on sale, and buy several. Several being 10 or more.
Get some Speed Loader Carriers. For IDPA you are limited to 3. Shooting other matchs you may need more. One of Pizza Bobs revolver converts just made an order with me for 5 carriers for his 686.
If you are planning on a Moon Clip Conversion you will want one brand of brass or you will be doing a lot of sorting. I have been shooting 38 Special ammo for the last 45 years, and have a couple 5 gallon buckets of cases. I end up sorting out RP and Federal for my brothers Moon Clip revolver. Not all brass will work with moon clips.
Good Luck
Bob R
Big Creek Speed Loader Carriers