The best source for unfired brass is
Silver State Armory The small primer brass will produce more consistent velocity, and SSA brass will last longer than Remington. The Large primers also show flat primers at a lower pressure than the small primers. You will have to back off 1/2 to 1 grain of powder when using the large primer cases.
I've had zero problems with CProducts 25 round mags, and the 10 and 5 round CP mags are good too. Avoid the straight 17 round CP mags, they have a reputation for jamming, but after replacing the follower with a magpul, both of my 17 round mags have been flawless.
44mag has good prices with fast shipping.
PRI mags can be loaded a little longer than the CP mags, but they aren't any more reliable and they cost 3x as much. Barret makes 30 round mags that are good too.
I've had the best results with H322 powder under 110gr bullets, both Hornady Vmax and Sierra Pro Hunter. Your ER Shaw barrel with the SPC2 chamber should max out about 29.5 grains at 2650 fps in a 16" barrel. I load them to 29gr and get 2610fps from my KoTonics lightweight barrel. Yours was built to replicate the dimensions of the KT-68 without the chrome lining.
130 gr bullets work better with H335, and the Barnes 85TSX and Speer 90gr TNT work better with faster powders like RL7 or H4198.