3 years ago I rebuilt an MGB with a 3.5 litre engine from a SD1. Because it was registered prior to 1973 it was classed as historic and so was tax exempt and cost £99 a year to insure fully comp through Footman James.
Note this - the DVLA go on the whichever is older - the body, or the engine - the engine was from a 1985 car, but the car was Nov 1972. More on that later.
If your landy isn't going on a public road buy whatever you fancy, bodge any repairs and leave it in your woods - no tax or insurance needed. I'm sure a friendly neighbour will sell you red diesel.
If it is going on the road, I'd buy a pre 73 restored/ish series I/II and it will have no tax, cheap insurance. I might even buy a restored rolling shell, or a crappy one and restore it myself - then I'd stick a later diesel engine/running gear/interior etc in there from a wrecked defender/disco. It would still be classed as a historic vehicle and thus tax exempt and cheap to insure.
If I did not have a landrover already that's what I would do.
Plenty of people do custom landies for off roading - you can just about do anything mechancially you want with them. Mind you I'm good with cars and welding.