The 625 is the king of IDPA revolvers because reloading is so fast, just throwing a moonclip into those big .45 chambers. IDPA split the revolver division into Stock and Enhanced a year or two ago just so a 'normal' revolver could have a chance.
As far my rig goes, I'm using a Blade-Tech paddle holster (about $65) and I'm pretty happy with it. It feels a little top-heavy and wobbly to me, but then all my other holsters have been IWB. I see their web site says the paddle has since been redesigned "for added stability", which sounds good to me. I might have bought the Fobus for $25 if the model for the GP100 had existed then, even though it has goofy sci-fi-looking 'vents' for no apparent reason.
My speedloader carriers are old-style 'split-six' snap cover leather ones that I found on Gunbroker as a package deal with a couple of the Safariland speedloaders I was looking for. There are better, faster open top ones out there.
Those Safariland speedloaders I chose because I heard they were faster than the HKSs, and they are. HKS, for some reason, is the only brand I ever see in a store. They work fine but require twisting a knob to release the rounds. The Safarilands, on the other hand, release by simply pushing the loader straight down onto the cylinder. The Safarilands come in Comp I (compact), Comp II (slightly more prominent locking button, the kind I have), and Comp III. The Comp III has a long handle and is spring loaded so it shoots the rounds into the chambers; it's what the racers use. There's also Jetloaders which I gather are much like the Comp III, and several other rather rare or expensive or out-of-production brands [one these is the Dade, which I just noticed somebody is
selling a bunch of on eBay].
Oh, yeah, there is also the Maxfire, which is one piece of molded rubber, sort of a round Speed Strip. The only people who buy these are cheap dumb people like me. Then they go buy something else.

In my hunt for Safariland speedloaders, it took some digging before I was sure which model was right for my gun; Safariland's site was not very clear. I know I'm not the only one with that problem, as my revolver came with one that was the wrong model! I wondered for quite a while why it worked so poorly. So anyway here's the results of my visits to the websites of various vendors:
Safariland Speedloader ModelsJ-N1: S&W N-Frame,27,28,38,357
J-S5: S&W N-Frame,29,629,624,44; Colt Anaconda,44 Magnum
J-C7: S&W 5-Shot,38 Special; S&W M60,357 Magnum,J-Frame, Ruger SP101
J-K2: Taurus 66,669,689; S&W L-Frame,10,12,13,14,15,19,64,66,67,68
J-GL8: Ruger GP100; S&W L-Frame,581,586,681, 686,357
J-D9: 38 Special,Detective Special,Agent,Cobra,Commando