[Doctor Who] How much of a TARDIS is essential to its function?

While we hear of the TARDIS having engines that are implicitly essential to it working, we’ve also see a TARDIS work without the rest of the machine.

“The Doctor’s Wife” and “Inferno” show that a TARDIS is capable of operating as just the console, which would seem to imply that they’re just a power source to allow the console to do its thing and move the whole ship around, or to allow for the pilot to do silly things like tow an entire planet one second out of phase.