Can I have Your "Two Cents" To Start Off Right? (BUILD)
 

So, I mainly do vintage builds and just got a newer Scattante R-660 for long distance biking. I love working on them and having a nice, finished product. It took a decent amount of time to learn the little things like "Schwinn sold out to Walmart, so avoid..," "body welded derailleur hangers = better built bike," "carbon fiber & titanium bikes are lighter but ride rough," etc. I want to do a new MTB bike to ride trails. What are some things you wish you knew before your first part purchase/build??

That one is never enough! Suspension on a MTB negates the rough ride a Skinny tire bike absorbs with stiff material. Steel is still much loved and appreciated, especially if you are fond of the vintage bikes.

I too ride some vintage MTBs, an 88 Nishiki Ariel for MUP riding, a 92 raised chain stay Nishiki Ariel on trails, just sold a 94 steel Rockhopper and sold a 88 Univega Diablo. With possibly a few exceptions, the geometry of vintage bikes can not compete with the newer designs. Much more stable when you need it.
Wish I hadn't gone "cheap" on the first new bike, landed up buying a dual sus only 7 months after buying the 05 Rockhopper, then some years later, bought the Stumpjumper that I should have bought the first time.
Best upgrade, getting rid of entry level shocks, going to a Fox F-100 (now a Talas).
If doing a build and need a part, going late model, high end used (such as XT level or X0) is much better than new low end.
Buying a complete bike is way cheaper than building.