If mobility is an issue mech and armor. 1-2 mech per 1 armor.
If you have an army already advanced that is mostly infantry, strait armor to catch up and hit.
If army is next to you, inf arty inf, inf arty inf, inf arty inf….
If you have difficulties doing mental math just follow this general rule of thumb for all axis and allies boards. Germany generally buys mech and armor in global. Only for defending later in the game do you get inf and arty.
You could buy all artillery for germany round 2 then plan to attack Russia on round 7. That is a much more conservative approach which is fine.
In aa50, revised, and classic. Armor costs 5. For classic buy armor the round before you attack, that is it. For revised and aa50 you generally end up with lots of tanks, because tanks are 5 ipc… sure you want infantry as cannon fodder.
With uk in revised and aa50 this is more important, 4 inf (1 per transport) 4 armor is ideal, if you cannot afford, 4 inf 2 arty 2 armor or 4 inf 3 arty 1 armor is really close to the ideal attack probability wise… if you expect all your units to die after 1 round of combat 4 inf 4 arty.