Really, it depends on your strategic location and your playstyle.
If you are active enough and prefer artillery, infantry is fine to use as meat shields in front of artillery or later on SP artillery). Usually if you invest in that early on, you won't need as many offensive units, diminishing need for motorized infantry. However, if you focus on planes (tactical bombers to be precise), motorized isn't a bad investment because of its speed in capturing provinces bombed to the ground (although really, you should use armoured cars for that). Mechanized infantry is alright to use late game, but don't bother before you get a level 3 barracks. The speed and cost isn't worth it until you can mass produce them and your industry can keep up. Usually you shouldn't really focus on infantry anyways with this doctrine, since your starting infantry units are more than enough to last until you cacn really focus on mass production.
Just to note, commandos and paratroopers have buffs as well, so it doesn't hurt to use them as elite infantry (although for paratroopers, I find their status as elite infantry more useful than their actual job). You usually don't need an anti-infantry ground unit, as your tactical bombers and artillery can deal with that pretty easily...
Just my two cents ![]()
