[Q] Begining Dev custom rom

Hi
I'm starting to dev a custom rom for my galaxy s3 i9300 and i have somes questions if somebody can answer me :)
i would like to know what firmware can i take from samfirmware ? specific version or i can choose ?
I'm in Europe (FRANCE) .
Second question i want to dev on kernel siyah...
Something to do for making my custom rom compatible with kernel siyah ?

EDIT 1: i found answers for kernel but still waiting for the firmware stockrom thanks

Thanks for reading and i hope you'll help me :)