I fitted a 200tdi into my 19j 90. Pretty much an unbolt, bolt up job as my 200 came from a Defender [ high turbo ] and the 19j downpipe fits straight on . Only welding was to lower the rad frame mounts for a neater bonnet shut line.
It will be a Disco 200tdi as defender engines seem to be silly money.
If using a disco 200 there is more to it due to the lower turbo position but parts are available. Another option is to adapt the 19j manifold to fit the 200 head, then you can keep the 19j exhaust system.
i would be fabricating a stainless down pipe and complete exhaust system so thats not a problem. i did look into swappint the 19j manifolds over but it all looks a bit sketchy.
One thing not to keep is the 19j rad as it will overcool a 200.
Didn't think of that
You are right to think an intercooler does little at low speeds [ we generally do not fit to 4x4 trials trucks ] However even when moving at 20mph it is working to cool the charge air going into the engine improving power/economy. [ In my opinion 19j's would have been much better with an intercooler leading to lower charge air entering the engine keeping head temperature lower and so giving longer engine life.]
See 19j to disco200 conversion in sticky section top of forum page.