How serious/competitive do you want to be, and are you looking at Stock Service Revolver (SSR) or Enhanced Service Revolver (ESR)? My interest is SSR, so I can't help you beyond that, other than to say you might as well pick up a S&W 625 and be done with it.
Here's my stuff:
I use a S&W 686 4.2". I've also used a S&W Model 10 on occasion but prefer the beef of the 686, the extra front weight helps me point it. I sort of oversteer the M10, but the M10 has a better trigger. Any of the old S&W revolvers are a good investment and if you don't already have a gun, I highly recommend picking up an used older S&W revolver. If you don't care about shooting .357, Model 10s, 14s and 15s can be found for excellent prices and are amazing guns.
For ammo, I shoot 158gr copper-plated flat noses with 3.4gr of Bullseye behind them. I find it shoots to point of aim with good accuracy at typical IDPA distances, has minimal muzzle flash and almost no recoil. The profile of the bullet also makes speedloaders easier to use. I'm tinkering with downloading it even further but haven't really settled on anything yet; taking it down to 3.0gr almost makes it kick too little and it feels kind of weird.
I tried some lead HBWCs that I use for 'accuracy' loads in Bullseye-type shooting and they gum up the speedloading process. Even though they're the most accurate thing to shoot the time losses aren't worth it.
The speedloaders are Safariland Comp IIIs. I use IIs with the M10. The IIIs are so much better, way faster to insert and more positive ejection of cases into the cylinder. The Comp IIs aren't bad but if you have a competitive bone in your body you will likely look for Comp IIIs in no time. I carry the fourth one in my concealment garment and use it to load before the stage.
The speedloaders are in North Mountain IDPA Speedloader Holders. I bought them from Dillion Precision.
I've never seen anybody else using these, not like I've seen a lot of revolver shooters in IDPA but you get the idea. Anyway, they're awesome. Open-top means no fumbling with snaps, the loaders come out super slick and are well-retained (I fell on a stage and they all stayed on my belt!). The back has a clasp that lets you snap 'em on and off the belt easily. They are expensive but I love 'em.
The holster is a Blade-Tech 5" 686 holster, nothing fancy. It does what it's supposed to do.
The belt in the picture is an inner duty belt from my job but I also use the inside of a CR Speedbelt sometimes (it's being used for my CZ75 right now). The CR belt is better but I'm kind of lazy and get annoyed by switching the equipment back and fourth off the belts so the 686 got the Uncle Mike's inner belt. It's perfectly functional. Make sure your belt is plenty stiff and won't let your gear wobble around on your waist, it can cause you to miss a speedloader in a stressful moment.
As far as anything else you might need, I strongly recommend getting into reloading if you're going to shoot these matches with any frequency, you'll recoup the cost in savings pretty quickly. Also, a loading tray for your speedloaders can be handy, but not all clubs have space to leave it laid out, so make sure it's ok before you pick one up.
It came to me with Hogue rubber grips which work well when the hands get sweaty in summer time matches. It had been given an action job that was just a trifle enthusiastic since it tended towards light strikes. I fixed it by inserting the cup of a spent primer over the tensioning screw as it appeared to have been shortened to lighten the spring.
