DerrickRL "Got fault code for Crank sensor. Replaced sensor, car started but same problem. Replaced sensor again"
ONLY a Bosch CPS will work correctly. If you used any brand other than a Bosch expect problems.
A know good RCM is always a good place to start as is a known to be good battery.
Throwing parts at is an expensive way to fix a problem; Better to get the car scanned for diagnostic codes and have a starting place to make repairs.
Is the check engine light on? Then there is a code. Often a bad RCM will not activate the Check engine light nor throw a code.

Again from you: "'m now thinking faulty O2 (Lambda sensor), clogged/collapsed Cat', or possibly blocked fuel filter. But where do I stop chasing the fault !!!!" all of those problems you mention will have a diagnostic code. You need to scan the car, before buying more parts.

PS. no programing of RCM required. You just remove the old one and install the new one. However, the "ECU" properly called the PCM, Powertrain Control Module, does require programing; It can not simple be replaced with another as the VIN is specific to the car and it must match up with the SKREEM.