I'd like to play as non-aligned Poland with its onw faction. Can somebody give tips about firts couple years of campaign? The most important technologies, building order, divisions templates, etc?
An advantage of hitting Czechoslovakia is that they virtually never get guaranteed by the allies, presumably due to that being very detrimental to Germany's buildup.
Why did you go fascist before justifying? Total waste of time, with a nation as flexible as Poland. Just pick up the Revanchism focus and go to war as a neutral. Poland probably does better not to pick an ideology early on - keep neutral and faction-free at least until the War breaks out. Only reason to do otherwise would be if the plan is to align with Germany, but OP stated we considered a Miedzymorze scenario.Almost never is not the same as never. In my most recent Poland game (historical focus), I flipped Poland to fascist and started justifying on CZE. French guarantee came by return mail.
For starting production in singleplayer and multiplayer, I usually go
3x Inf weapons
1x towed AA
1x support quipment
4x Close Air Support
and then import 1 aluminum and ignore oil/rubber (the production malus isn't really noticeable, you start with 2 oil). The summary: don't neglect some air force. Air bombing is very valuable in the current meta of HOI4.
As for the looming war with Germany, Infantry+towed AA is perfectly fine for holding defensively. With level 2 or 3 forts along the rivers surrounding Warsaw, Lodz, & Krakow, nothing short of a huge amount of air superiority will dislodge you. Entrenchment generals & engineers & doctrines, 3 defensive/combat ministers, encryption bonuses, and AA preventing enemy air superiority maluses, + some unit experience will give you divisions with 700+ defensive stat. Your CAS will eat tank spearheads.
Also don't forget to snag construction tech 3 by mid 1937. The Polish focus tree has 2 industry research focuses unlike the generic. Stack with defensive focus, economy laws, and the industry minister to produce a military factory in just under a month. The 4th research slot can wait a bit. With a good industry plan, you can reach 50+ factories before war breaks out in 1939 and have some to spare for a small mechanized attack force to pincer where it counts. When research slots are tight, motorized rocket artillery is your best friend.
But wouldsn't german's fighters eat all my CASs if i well build only them?
Detection early on in the 1930s is very low. Most countries won't have the radar range or the full tree of air doctrines necessary to be a big threat to your defensive airforce. That said, having a lvl 1 radar station just behind your forts is not a bad idea for your own interceptors.
And now i'm planning to add 1 support AA.
Why did you go fascist before justifying? Total waste of time, with a nation as flexible as Poland. Just pick up the Revanchism focus and go to war as a neutral. Poland probably does better not to pick an ideology early on - keep neutral and faction-free at least until the War breaks out. Only reason to do otherwise would be if the plan is to align with Germany, but OP stated we considered a Miedzymorze scenario.