94 and 95 Defender 90 NAS radiators have oil coolers and will fit. your specific Air con condenser configuration will dictate the need for an extended grill or not though. upper and lower Hoses will be different than the 4.0 though I think (I can take pictures of a 4.6 Gems in a 130 Tuesday give you some ideas, no oil cooler though as its not really needed.)
just food for thought.. having an integrated oil cooler means that the oil is going to be at the same temp as the coolant (195 degrees), separating oil cooling from direct contact with the radiator is going to allow it to cool better (in front of the radiator or below it as I did with my trans cooler). So it would be a better design to separate oil cooling from Engine cooling IMHO.
No idea about OAT coolant though or why its needed. Seems most states still allow the green stuff to be simply disposed of. Simple and cheap to flush.
No idea on the piggy back to the ECU.
Coolant should never get into the combustion chamber, if it does you have bigger issues with a cracked block or head gaskets, etc. on my 110 I switched to a 92 RRC motor with an auto and cats. Coolant type is the least of your worries there.
if your worried about spares, buy a couple of D2's with good engines, they aren't that expensive and probably cheaper than buying the parts peicemeal