Hi, I'm currently making a 489 RNZAF sqn TFX. My best suggestion would be the sam publication modellers datafile 6, The Bristol Beaufighter.
Its a very well detailed book that covers the Beau in depth. From the listing for the various Hercules engines it says that the TFX, MXIc and Mk21 all used the Hercules XVIII (At some time or another).
As far as the external differences go, it certainly has the Auto pilot in front of the cockpit windscreen, the early production DAP Mk 21 carried standard 2X4 wing mounted weoponry. the late production had the 2X2 wing mounted .5in machine guns.
As far as photos go, the book has a huge number of colour photos of a restoration project at Duxford, a TFX at Hendon, and a TFX in Ottawa Canada.
Hope I've been of some help.
Cheers