As previously posted the reinforcement tabs are to reinforce the front hole in the receiver that holds the upper onto the lower. The reinforcement tabs are square with one corner cut at 45 degrees. They are TIG welded across the top and down the 45 on the inside of the lower receiver.
As you can see there are many options for stocks. Personally I think starting an AR stock adapter and a collapsible stock will give you a solid stock to start with. Do you want a long or short length of pull? Personally I like a short LOP so that the weight and energy of the gun firing is closer to my body so I have more leverage to control it. In .45 caliber with the factory upper there is a lot of energy released during a mag dump. A good stock and shooting position can control that energy much better than a 3/4 "chicken wing" positions with a flimsy stock. Depending on wear, I have heard several owners talk about the original wire stock collapsing during firing. I was lucky, my first M10 came as a package deal with an aftermarket upper and AR stock. I don't know as I have ever used the original wire stock ever. Again congratulations on the transfer. Just because someone else thinks a configuration and/or stock combination is "the best" doesn't mean that it will be the best for you. Buy used at a good price. If you like it, keep it. If you don't, sell it for what you paid for it.
Scott