Amen to DreamPi. If you get a Raspberry Pi, you'll need a SD card that matches your Pi for the DreamPi OS. I also use my Pi (with a different SD card) for Kodi, but that's another way to justify spending money on a Pi! Hopefully your Dreamcast modem is still there (years ago I bought a used one for $10.00 but it was not there...odd).
Before you buy, here are your options...
a.) Buy a DreamPi kit from PCwzrd13 IF you do not have a Raspberry Pi, USB Linux modem and a LVI, or...
b.) Buy a DreamPi Modem from TacT if you have Raspberry Pi, or...
c.) Buy a LVI from PCwzrd13 (which requires a 9v battery or two 9v batteries depending on PAL region).
d.) But if you feel you can make one, PCwzrd13 has a video to guide to help you (link below).
About those options, they BOTH come pre-tested and with a warranty. Option "a" is the best option because it just works and you will never have to worry about batteries. Option "b" is also a must if you already own a Linux friendly USB modem but you must be sure your USB modem will work with Linux.
https://www.youtube.com/watch?v=lPvNQAi-yeM