That’s a solid purchase list.
If germany clearly prepares Sea-Lion:
RUS: 7 inf + 1 art + 1 bomber (the range gives a lot of options on all fronts early on, and those russian bombers are so biiiig)
UK: all inf (3 inf defend better than 1 fighter)
US: atlantic subs, probably a destroyer. but will vary from game to game.
if germany clearly prepares Barbarossa:
RUS: probably some inf + some art + fighter
UK: depends on the outcome of the naval battles, but still defensive at first (beware the hidden sea-lion!). And air force (which is defensive as well, so hey)
US: also depends on the situation, but probably pacific ships.
France: 3 subs :D
the others, about the same as above, i think