Subs are by far the most cost efficient naval attack purchase. Surprisingly, they do okay in defense as well since hit points are so important. Japan should only buy subs if usa purchases pacific naval units round one. Since subs have more attack than defense, the usual dynamic is that two fleets are 3 spaces apart. The stronger navy will try to move to two spaces, either through enough defensive strength or with destroyer blockers. The other side has the choice of building up defense, countering similarly with blockers, or retreating.
3 transports, 3 infantry is an all around flexible and powerful round 1 purchase. If UK attacks japan sz37, this strongly suggests KJF and a buy of 1 trans, 3 sub 1 inf might be more appropriate.