You won't find a 3.5 RRC with a hot wire system that was later with a disco.
In the 10 years I had my 3.5 all I had go wrong with the engine management was the ECU failing giving difficult starting (over rich) and a ECU coolent temp sensor failing.
What to look for, blue smoke from the tail pipe, oil fumes from the rocker cover, remove the two rocker rocker cover breathers and check on condition the both they should be clean especially the foam one at the rear also condition of hose's, plug leads oils/fulids etc the usual stuff and no rattles.
By now I would think most of the important stuff as been replaced, best to start the engine from cold ie. over night, if the engine has been pre warmed before you get there this can hide an few problems.