Again, it all depends on what nation you're playing. Some, like the USSR, have so much manpower it almost doesn't make sense not to attach arty and AT to infantry, unless you need something really fast. With others, notably Germany, one must balance the need for a large number of divisions fairly quickly with the restrictions of a limited MP pool. Often the best way is to produce a certain percentage of units with brigades, (as Germany, I produce 1.3rd of my inf as inf-A) mostly inf-A, though it can be worth it to consider attaching an eng bde to a few divisions of fast, light tanks in order to build an exploitaion force.
By all means build mech and mot and armd. Armour are great as shock troops, with mech to provide the backbone of your mobile force. Early on, mot can be used in the role of mech and later, surviving mot units can be used quite effectively in some of the secondary theatres and/or as part of the mobile reserve in Western Europe, ready to reinforce beach garrisons.