I personally think buying German naval units in the Baltic are not that worth it because the Allies should do the following:
Russia should constantly harass Germany and not allow them a foothold in Karelia. If Germany moves into Karelia without sufficient forces (which happens when they purchase naval units), then Russia takes out Karelia thereby weakening German land forces. With weakened land forces, Germany loses the threat against Russia
UK builds the Aircraft Carrier, transports and fighters (R1, R2) then corners the German fleet in the Baltic (R3). Either Germany attacks the UK fleet and will lose its fighters and fleet in the process. Or UK attacks the German fleet with minimal losses if the fighters wisely choose to abandon the Baltic fleet. Or UK attaks with some losses if the fighters stay on (but its worth it to kill the German fighters)
US loans UK the 2 transports and destroyer as fodder in case the German navy attacks, while they keep building transports into Africa.
Then you have a 3 on 1 against Germany with no Japanese help against Russia until R5, R6. Anyways, my 2 cents