If you already have the EchoLink software on a PC then the built in help has a lot of info. So does their web site. It tells you how to configure the software and hardware for node use (EchoLink calls this "sysop mode"). and has information about radio interfacing. I would suggest reading all of that first.
My advice is avoid any interface that uses VOX.
VOX sux. You need to use a serial port for COS or your node will either cut in and out or send vox delay hang time over the internet. Either of those conditions make for a crappy node. Most modern computers don't have real serial ports so you can use a USB-serial converter dongle.
Some radios such as the Icom IC-2720H, Yaesu FT-7800R and Alinco DR-235T have a rear accessory connector designed for packet radio use. his makes it easy to connect the radio to a computer. The connector has pins for audio in and out, PTT and COS. I think the COS line put out 5 V on all those radios and I never had problems with that voltage being too low for the serial port. If I remember right on all of those radios you can even leave a mic connected because if you key the radio via the rear connector PTT line it disables the audio from the mic connector.
I built my own interface and used it on all of the above radios over the years. All you need is some connectors, two pots, some caps and resistors and a transistor or optocoupler for PTT. You should be able to find different schematics and plans online.
