You can't really "overkill" anything, so a while a 58 certainly isn't necessary to kill deer sized animals, it's fine to use one...or even a 62.
Everyone has their own ability to handle recoil, but the stock shape and especially the buttplate shape can certainly impact how much of it you feel. I built a fullstock Hawken in .58. and with the load it liked, which was 100 grains of 2F with a tightly patched .575 ball, it definitely had a push which was increased due to the rather narrow and curved buttplate of a Hawken. My .62 early Virginia with a wide and flat buttplate with the same load is mild in comparison.
It's not hard at all to find whatever you need to go with a .58. Many of our fine online muzzleloading supply shops carry anything you would need for them. Track of the Wolf is a good source, but there are many, many others.
I do think that the number of production manufacturers making 58's is more limited than .54 and down calibers. Pedersoli has a few models...The Kodiak double rifle is one although that's a fast twist. Or if you like the look of military rifles, they have the Cook & Brother or the Mississippi.
I know you said roundball, but my Pedersoli Frontier .54 with a 1-66 twist shot patched round ball OR the Hornady Great Plains 425 grain bullet with great accuracy. So does my Pedersoli Rocky Mountain Hawken. That bullet would be great elk medicine. Or you could probably have a .54 rebored by Bobby Hoyt to .58 with proper twist and deep round grooves for roundball. Quite a few options to think about.