First off I want to say it is NEVER a good idea for the Axis to attack Strict Neutrals. For one, you give the allies all of S. America and the neutral armies in Africa for sure, if not some of the middle eastern and Afghanistan armies.
Secondly, the allied attack of Neutrals is pretty much the only GOOD strategy for breaking into Europe. But you need to do it wisely.
A) Control the Atlantic. Without this, you will give the Germans more income.
B) You need to take Turkey AND Spain simultaniously, to prevent the Axis from annexing one and getting a large army boost.
C) You want control of Norway/Finland, again, to prevent an army boost and also because it’s just plain easier on you if you have it.
Attacking the true neutrals gives you advantages.
A) Spain is in range of E. USA via SZ 91, and you can move your transports all the way back to SZ 101 with the naval base in Gibraltar.
B) Spain is defended by 6 Infantry, W. France and Holland might be defended with more.
C) Armies in France cannot hit Spain due to W. France and S. France (Tanks can, but I am talking infantry/artillery here.)
D) Spain is worth 2 IPC and thus you can build a factory there. (If you plan to ever liberate France, and I don’t know why anyone would be that stupid, this factory might come in even more handy than it normally does!)
Turkey is not something you really need to worry about “holding” it is more an attack to prevent Germany from getting 8 more guys. Once you “clear it” it’s a matter of a British complex in Iraq to keep it cleared so that the Germans cannot use it as a short cut.