Welcome to SPCR.
Don't have answers to all of your questions, but here's a couple thoughts. First, you don't need to underclock/undervolt an i3 to make it silent. Skylake does a great job of adjusting both the core voltage and frequency at will based on the processor load. Here's an overview.
Practically speaking, if you are web browsing, the CPU freq and voltage will jump up during page loading and jump down after completion. Leading to your CPU mostly idling, consuming just a few watts.
So, if silence is the goal with a lightly used i3, all that's required is a decent cooler and some airflow through the case.
If, for the sake of experimentation, you do want to mess with undervolting and underclocking, it's mostly research to find a motherboard that has those options open. For undervolting, you'll want a board that allows a negative core offset voltage. Z170 boards are the most likely to have the option open - you really need to read through the manuals. Or hope someone will pipe up and and say their mobo has it.