Not sure there are any specific diagrams. A 12 valve is a very basic engine and only requires 1 or 2 wires to run. You need 1 switch on constant hot to the injection pump solenoid if you use one. Alternately, that can be done with a push/pull cable. The other power lead is the starter circuit. You'd need to work out the engine monitoring wiring such as oil pressure, coolant temp, tachometer, etc and add at least a turbo boost gauge which is normally mechanical. Another useful gauge is EGT which is highly recommended.
No - not in the least. Unless you need a diesel or have the extra $5-10k potential service work a 1/2 ton gasser will be fine.
Now, if you aren't afraid to drop some cash a diesel is a toy that never gets borning. For the Dmax's all the problems are known, its just a matter of whether the work was done to fix the OEM and mileage/age issues and whether it was done correctly. Lots of half ass mechanics they can work on diesels - do not believe any of these people unless their parking lot is primarily full of diesel Showbox vehicles.