Hmm,I will recommend the 2.5 litre as the road tax for a higher CC diesel engine is more expensive.I think you can retain the gearbox but the gear ratio might not be suited for the different torque this engine produces.Usually they transplant the engine together with the gearbox that came with it.

The ECU must also be changed.I think if you leave your car idle for that long,alot of electrical stuff will be dead by now and you will definitely need four new sets of tyre as the tyre would be unevenly round now.