okay so... I'm still relatively new to mountain biking. I rode bmx for 10 years and have commuted on my SS CCX bike for 2 now. I definitely have the mechanical aptitude to put a bike together, but I don't really know where to look for the parts, or, more importantly, what parts to look at.

Can someone point me in the right direction?

I want a steel, 26 or 27.5, fully rigid SS with mechanical discs and tubeless tires. Feel free to suggest any parts at all.

Stem? (and what actually attaches the stem to the fork? I guess this I don't know)
Discs? (Mechanical preferred)
SS hub for mtb??

the other stuff, the seat, bars, grips, etc., I got pretty well figured out. but with this other stuff, I just don't really know what brands make what and where I can buy them at the best prices.

I wanna keep the whole thing under $1200 if possible, but if I have to go a lil over I'll make do.