@menuhin: I actually like Matias switches -- Click and Quiet; Quiet Linear, not so much. They are okay in the KBP V60, but I don't like the overall effect of Matias keyboards, which remind me of Fischer-Price toys. I think that Matias uses a polycarbonate plastic for its cases, and the glossy finish shows fingerprints.
Although I like Matias switches (but not Matias keyboards), I like vintage Alps much more, and I applaud your interest in doing a custom build.
The sound of a switch depends upon the switch itself and the keyboard in which it is installed. However, in general, clicky Alps are not as loud as clicky Matias switches, tactile Alps are usually not as loud as clicky Alps, and damped Alps are usually not as loud as clicky or tactile Alps. If you bottom-out your keystrokes as I do, and if you want quiet, then you may wish to go for a damped switch (although I find the damping to result in a somewhat mushy feel).
My V60 Matias Quiet board is almost as quiet as my silenced and lubed Topre-switch RF87U or HHKB Pro 2.
Take a look at the Alps SKCL/SKCM series wiki on DT:
https://deskthority.net/wiki/Alps_SKCL/SKCM_seriesThis tells you a great deal about each type of Alps switch, and it has links to the keyboards that have the various kinds of switches.
You may wish to try some vintage boards to help you decide which switch type you want for your custom build.
Best wishes!