As far as recoil goes...the 500 will recoil quicker and a bit harder compared to the slower push of the 45-70. The 500 uses quicker powders than the 45-70 and thus the acceleration is a bit quicker feeling more like a quick jab than a push. I found that the muzzle doesn't rise as much on the 500 compared to the 45-70, but that's my personally experience. With the shorter barrel you might have a bit more recoil, I'm not sure though. Mines the standard length barrel.
As far as powders go, what do you define as medium? H110 is for full house loads, DO NOT REDUCE H110. It has some of the highest velocities out there, next to Lil'Gun. A quicker powder with a lighter charge would work best for a middle of the road deal. I personally use Long Shot for middle loads. I can push a 400gr lead slug at 1400-1500 FPS compared to the full house of nearly 1800 (It can be pushed harder, but I've found this to be the sweet spot accuracy wise). For lighter loads I turn to trail boss, 14.5gr's behind the same lead slug does around 1000 FPS and the recoil is almost nonexistent. There is no problem is using H110, it will just be a hot load. In fact some of my most accurate loads with the 500 have been with H110.
As far as primers go, Large Rifle is what new brass is designed for. When the cartridge originally came out it was using Large Pistol primers, but they changed in 04-05 I believe. So as long as you're using new ammo/brass, they would be using Large Rifle primers.
Edit: As far as brass goes, I've heard Hornady is a bit thinner than Starline but I haven't really checked myself to confirm that. I use both Hornady and Starline, but keep them separated. The Hornady's are into their 7th reloading (all hot) and the Starline are into their 5th (all hot) and neither have any stress marks on them.