Yes.

Mandrel bends use a block or other sort of big heavy object (called a mandrel) which the pipe is bent around to keep the pipe diameter constant at the bend. Crush bends just bend the pipe around itself which crushes the pipe in at the bend and thus makes the inner diameter smaller.

Ideally, you want mandrel-bent stainless pipes. Diameter is a function of what sort of setup you plan on doing in the future. If you're going to just stick with bolt ons and leave it at that, 2-1/4" piping is the best choice. If you plan on doing a more aggressive build with cams and such, 2-1/2" pipes will benefit you.