Percussion guys may still want to split an order of real BP with you, worth asking. You can mix granulation types as long as the minimum is met and the max isn't exceeded, so everyone can get exactly what they want.
With a .58 I'd start at about 80 grains of 2f. I do not own a .58 and haven't shot one, but I don't think you'd want to start much lower than 80, I recommend to start at 70 with a .54 based off my own experience so I think 80 should be in the ball park. Don't worry about fine tuning the sight quite yet, just make sure it's hitting paper. Shoot a group, go up 5 grains to 85, shoot another group, go up another 5 grains, etc., etc., until you find the best group. You'll find a point where groups shrink and then start to open back up. Of course don't exceed max volume charge. Then use the amount that gave you the smallest groups to dial in the sights where you want them. The optimum powder amount for accuracy can/may differ between a patched round ball and a conical, so each would have to be checked separately if you plan on possibly using both. I'd start at 25 to make sure on paper and then move to at least 50 yards. The difference in how a load will shoot at 25 vs. 50 really separates the good from the bad.
I carry a shooting/possibles bag when hunting all the time. A common theme with people just getting into BP shooting is they think they need a fairly large bag, buy one, and then wish they had a bit smaller bag once they get some experience. Just FYI. I think a 6" to 7" bag is best, but everyone is different. I very much prefer a stiffer bag that holds it's shape well vs. a soft pliable one. I like minimal when hunting. I load the rifle and then carry in my bag a small homemade field flask that holds 5 reloads of powder, 5 lubed patches in an old cap tin, 5 round balls in a ball bag, a jag, a nipple wrench, a decapper tool that has a nipple pick built in, and a container of damp cleaning patches. I wear a straight capping tool and a homemade antler powder measure around my neck.
You're going to want a priming flask and touch hole pick with a flinter too, probably an extra flint, and perhaps a "cow's knee" for putting over the lock in foul weather. Many just tuck it under their arm in foul weather. FG or one of the other flinter shooters will know better than I do, I have solely used a percussion.
I also always bring my main bag with me, it is a decent sized bag that has most of what I own related to BP shooting, so I have things like a ball puller, etc. at least at the pickup.