Here's my thoughts... Listen to Pressbrake and a few others.. You are talking big power before you have experienced std power from a Cummins 6Bt.. I completed a conversion fitting a Cummins 6Bt and ZF 5-42 gearbox coupled to an LT230 into my 101FC. When I finished building it a took it for its first test run.. without the speedo connected along with a few other things... To be honest, I was not wowed by it like I thought I'd be.... Now I've got used to the shear torque from this engine, and have given the injection pump one full turn of the fuel screw to suit the intercooler I've added, I now can't get over how well it goes.. Assuming the Daf 45 engine was 130hp, and with an intercooler and increased fuel Im now guessing its at approx 150/160hp.. It flies! The torque is awesome (and smooth not like a course 4 cylinder), and it also seems to pull right up to 3000rpm. Do I want more power now? No. It does not need it. Really, it doesn't! It gets to 100kph very quickly, sounds awesome, seems good on fuel, and so far the trans etc is surviving (I did add an Ashcroft one piece center diff cross shaft and an increased capacity sump to help the LT230). Some facts.... The ZF 5-42 (5 speed, 420nm of torque) box is user friendly in a forward control but you'd need the US top shift to run it in a Defender. The Cummins is big, and it is a lot of work... Bespoke engine and chassis mounts, sump reversal inc oil pick up, serious machining to get the ZF to fit the LT (inc gear welding if you go down my route).. Props cut and shut, front springs beefed up, exhaust manufacture, rad mods or larger rad, sound proofing (LOTS!), and then you have the 12/24v issues.. (I've run 24v start and charge, running 12v for the vehicle. This can be done easily using two decent relays and a separate 24v charge light). You have a lot to think about, a lot of work to do, and if you are not a proficient engineer as you have said you are not, then you need quite a bit of money to spend... Pressbrake has hinted at this, and Im telling you the same.. Be sure you want to attempt this conversion as a £1500 weekend TDi conversion it is not! Good luck!
Oh, and here's a shot of the ZF to LT under my 101!