Cheapest method for completing an "it-works" computer with your K6-2/400 would be to get a PC-Chips M598LMR (BAT) or M599LMR (mATX). Those bring graphics, sound, modem, and LAN all onboard. All you have to add is RAM, drives, and a case. I've always had good results using those (although especially the BAT model makes a real mess of ribbon cables and slot brackets).
Regards, Peter