IMO these are the must haves for each nation (rares only.)
KV-1 (awesome tank with great armor, damn hard to destroy)
T-34/85 Guards (nice fast tank with great armor and good AT dice)
Sturmovik (plane decent AI and great AT attacks)
Churchill Crocodile (heavy armored tank with flamethrower, very hard to destroy)
Spitfire (damn fine fighter plane, this plane owns the sky)
Churchill AVRE (good obstacle destroyer, good for an offensive)
Veteran M4 Sherman “Rhino” (a tank with great manurability)
P-51D Mustang (damn awesome plane)
M7 105mm Priest (need heavy bombardment? look no further)
SS Panther (its a monster of a tank, gives me a nightmare as Russians)
Veteran Tiger (another monster tank)
ME-109 (isnt that great compared to other fighters but its vital in a German army)
Stuka (awesome tank buster)
All their rares suck ass. Their good units are infantry. However if you still want to collect the Japs, then any rare is gravy.
Also, if you are looking to enhance your game with your son a bit more later on, then you might want to consider looking into 3-D terrrain. It makes the game look more realistic and fun.