Manchuria is a very good buy, but not too early in the game or it will just slow you down. A first turn Japanese factory is probably the most common “rookie mistake” out there - a perfect example of the difference between looking one or two turns ahead instead of looking five or six turns ahead. It helps in the early, small battles for minor territories, but it hurts you in the big battles that really count - Novo and then Russia. The factory is mainly for buying tanks, and you need to get a good amount of infantry onto the mainland to support them first. A quick push will just stall out as you get close to Russia and your tanks will end up waiting on infantry reinforcements. This may already be basic knowledge to you - I don’t know your level of experience - but never go all or heavy tanks with any country. Try to keep at least a 3 to 1 ratio of Infantry to Tanks - I usually go 4 or 5 to 1 since Japan has plenty of planes for offense. I just mention that because a common rookie Japan strategy is to buy 2 factories as soon as possible and start cranking out all tanks - this is easily defeatable by a decent Russian player.
Here is what I like to do - I wouldn’t consider it too agressive or too conservative:
Turn #1: Buy 2 Transports and 3 Infantry
Turn #2: Buy 1 Transport and spend the rest on Infantry
Turn #3: Buy 10 infantry, save the rest
Turn #4: Buy 10 infantry, save the rest
etc…
This way I am transporting 10 infantry a turn to the mainland, and as soon as the ‘saved’ IPCs add up to 15 I buy a factory, which is usually Turn #5.
I will continue to buy the 10 infantry each turn in Japan, and then whatever is left over will get spent on 3 units at the factory - usually a mixture of tanks and infantry until you get to 45 IPCs per turn.
You will also want to buy a second factory a few turns later, either in Burma or more preferably in India.
A more agressive strategy would be to buy the factory on turn 3 or 4, and probably in Burma instead of Manchuria. A more conservative strategy would be to get up to 6 transports and 12 infantry a turn before buying a factory, and then sticking with one factory the whole game.
All three have pros and cons and their effectiveness depends mostly on your playstyle.