In a nutshell, cold smoking is smoking at less than ~85 degrees F.
The reason I asked what you intend to smoke is because if you're smoking stuff that you intend to hang, I highly recommend going with the #4.
As far as the accessories go, a PID is nice but certainly not essential.
They are great in that you can control temps very well, and with a good one you can program steps (different time and temperature profiles throughout the smoke).... that's extremely useful when smoking things like cured sausages.
Some use a PID so that they can bypass the stock controller and smoke at temperatures higher than 250 degrees.
If you're a Facebook member you can get a 5% discount on a Smokin-It smoker by ordering thru Smoin-It's Facebook store. Just "like" Smokin-It on Facebook and click on their social store link.
Here's info on bypassing the stock controller.... http://smokinitforums.com/index.php?topic=1287.0
There's lots of other great content on their forum.