You want some normal infantry purely for the very earliest phases of the invasion and for holding ground after you secure France/Italy - but you want your low MP divisions to be available for most of the heavy lifting due to lack of MP.
it depends on where you intend to invade and how early you intend to do said invasion.
I would avoid having a *ton* of mot if you're going into Nor/Ita because it'll hurt you in the bad terrain and questionable infra. Supply *will* be a problem if you were to go exclusively mech/mot, but it would also be exceedingly expensive for too small of a payout.
going for something in the ball-park of ~20 arm or mech divisions and at least 1-2 armies of 2xINF+attachments should be sufficient and doable, with any excess MP going into mech/arm or even HARM. If you use your navy and air forces effectively and make sure any major resistance is met by arm divisions you shouldn't have too many MP related problems.
If you were wanting to invade much earlier (say early 41' or earlier) you would need more tank divisions because you'll meet much higher resistance from Germany.