Are there any good kits or exact instructions/part lists?

you can grab a full kit here: http://atreus.technomancy.us/

but it's pretty easy to source the parts, see https://github.com/technomancy/atreus/ (which also has links to build instructions)

Are you using it for writing codes ?

honestly, I still struggle with it a bit (a month or two of casual usage, mostly English text, some C, some Python) and I've tweaked the layout a few times, but I think it's a lack of familiarity rather than a more fundamental issue like not having enough keys/needing several layers.