Gary Sinise really does look the part. He was my first choice. However, he's a tad old (not really overly so...I think Kelley was closing in on 50 during the series). I do think having McCoy noticeably older would be better. I like that dynamic of the younger hot shot and the older wise soul....
That said, Karl Urban can do a southern accent. He stared in a TV Miniseries Comanche Moon where he played a southerner.
Thanks to Ron Salon for the signature banner!