I have a remington 870 SPS-T with thumbhole stock and absolutely love it. The most comfortable gun that I have ever held. If you have your heart set on a mossberg, they make a thumbhole stock in the 835 as well.

When trying to find a turkey choke, you are going to have to take the gun out to the range and start shooting at paper / cardboard and see what works best. I would go and buy one choke and then buy a couple of boxes of different brands and different shot size and start expierimenting. Pattern a turkey shotgun can get very expensive very quickly. The Winchester Xtended Range HV shells are considered to be a very popular, good all-around patterning shell that runs around $30 to $35 a box. I would start out with a 3" shell with #6 shot...this gives you the added advantage of having more pellets in the shell and the #6 shot has more than enough energy to kill a turkey out to 50 yards.