Which engine to go for?
The overwhelming majority of Defenders have 5 or 6 speed manual, 2.5-litre diesel engines, this can be a 2.5D, 2.5TD, 200tdi, 300tdi and td5. Up to 1998 Land Rover fitted a TDi engine, between 1998 and 2006 they fitted a TD5 engine, and since 2007 Land Rover have fitted the TDCi (Puma) engine. Petrol versions include an older 2.5 petrol and 3.5 V8. You may find one with a 3.9 V8 or bigger but these will have been fitted aftermarket.
For extensive off-road use with lots of short journeys, I would most probably recommend a diesel, but for some other applications the diesel’s noise, harshness, lack of power and short service intervals may count against it. Petrol versions are not as reliable offroad due to water and electrics, how ever there are ways of water proofing the electrics for offroad use and many professional winch challenge trucks and trialers are v8 petrol. LPG is also available to help save money on fuel.