the selection guide is ok within it's scope, but it's a little bit outdated and it doesn't touch some low-cost solutions, that might be an option depending on your application's requirements. That's why I haven't guided you to this web site.
Calculating a load's inertia is very important for motor sizing. The effective inertia depends on parameters like mass, geometry, gear box ratio and so on. While you can calculate inertia mathematically, you also can use tools for motor sizing, that are provided by many motor vendors. NI is cooperating with Kollmorgen, so you can download their Motioneering tool from our website.