That's been the story of my life in recent years actually, building my own keyboards.

Yeah it's actually kinda hard to know where to begin because it just depends on what you are willing to do in terms of time and $$$. There are so many great resources out there.
My advice is to focus on an aspect that you really want to customize and leverage existing tech for the rest first. For example, if you really want to build a matias alps booard, I'd recommend learning to desolder existing switches and getting an alps switch to work. Or getting a led to work on a non-leded board. Focusing on one aspect of a mech at a time help me to take things piecemeal.
Now this for those of us who are trying to be reasonable somewhat

with $$$ and time spent wisely. I mean, by all means if you have the resource...