I have been riding my current bike (santa cruz chameleon, 7 speed, magura rim brakes etc) for about 8 years now with few upgrades so have decided its finally time to bite the bullet and build a new bike. I am a little out of touch with new modern technology advancements and genral shiny bike bits.

I’m going to buy a second hand Cotic soul frame (small) and build it up with 120mm forks.

My first big question relates to cranksets. I intend to run 2×10 speed, do I need a specific double chainring crank? Is the alignment taken up at the bb or is the crank design specific to take 2 or 3 chainrings? If I were to run 2 chain rings and a bashguard would I need a 3 chainring specific crank?
Also what crank length would you recommend?

I appreciate that you need to run specific 10 speed chain, chainrings and casstte but will a 10 speed cassette fit onto a standard 8 or 9 speed freehub?

I appreciate your help and no doubt will have a few other questions as the build progresses.