A lower is a lower is a lower.
Find the one you want, then start your project. There is no specific requirement for a "pistol lower." It just has to be a lower that has NEVER BEEN CONSTRUCTED INTO A FULL RIFLE.
That means any lower, stripped, complete, even complete with a butt stock can be used to make an AR pistol.
Caliber is up to you. They can be made in any caliber that you would want to use. The bits and pieces will be the same weather a rifle or pistol configuration.
Gas tube length will be determined by the barrel length. In .223 any length less than 10.5" will probably be better off with a pistol length gas system. They do run dirtier than the longer tubed versions, and weather a piston is worth the extra cost, is up to you.
I don't hunt with mine, but I suppose you could if you were so inclined.
Enough talk, here's some eye candy for you.