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.

Frame?
Fork?
Rims?
Tires?
Crank?
BB?
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.