I try to get a realistic composition and I everytime forget one-two things to buy (like ammo trucks or MANPADS teams etc
))
I haven't played all that lot of PBEM games, but usually I follow the pattern of equivalent units of tanks and infantry (meaning size). I also try not to have lots of arty and as on-board I often prefer SP mortars.
I don't use on-boards long range SAM's.
In one WW2 PBEM (small sale) I played I got a total of four light tanks and two 47mm AT-Guns (OK, since it was 1938 Czechoslovakia vs. Germany, LT-35 was still a tank compared to Panzer IIIa) and a lot of infantry (tried to play it realistically) against a German juggernaut of MANY Panzers. Ended up as draw though I lost no tank and last remaining Panzers were hiding between the village houses, praying for mercy
But it was a short game and I didn't have time for counterattack.