Where is the best place to buy the Diablo tuner? On partshopper you can get $50 off, but is the service better at somewhere like Modular Powerhouse or other places where they have you enter your current mods when you order it?
Good deals can be found on used Predators, and they can do all the same things a new one can. I have seen them for $200 and less on other Mustang sites. Your current mods do not alter the generic tune that comes on a Predator out of the box, however they do affect your vehicle's performance characteristics (i.e. A/F ratio, etc.) after loading the tune just as they affect your stock tune. A custom tune is the best way to adjust your specific vehicle for the mods you have or may add in the future.
Make sure you go with an authorized reseller if you want ANY support from Diablo. Most of these places that sell for under $399 are not, and Diablo will not offer support since these are sold under minimum advertised price.
The support from Diablo itself has been less than exemplary in my experience, however the support from a Diablo dealer from whom I received a custom tune was more than excellent.
Mossberg is correct in stating that used Predators are not warrantied, as is the case with nearly all used auto parts and electronic devices.