I cut my FREEPBX teeth using Cisco’s 7940’s mainly because as @BlazeStudios says, they are cheap and I wasn’t going to spend too much at that end of the learning curve.
It was difficult to get them going, I did brick a few too!! The information is out there, that’s where I got it and they worked perfectly well in my business and now are in a box in the basement as I’ve upgraded to SIP and don’t regret them as a learning tool albeit pulling what little hair I have out. I agree with Tom, you’ll save time and a divorce if you go with SIP devices.