I can run the whole show (ignition and fuel delivery) with an after market ECU, but would like to use factory ECU (Motronic ME7.1.1) to keep some features that can't be controlled with the after market ECU such as drive by wire (DBW) among others, also want the car to pass emissions when is needed.
A VEMS with DBW controller is ~ 500€ , most standalones can drive DBW meanwhile.
Most offer customizeable CAN messages and more. But yes its a bit of work to wire "everything"(tm) in.
Do you have OBD CARB readyness testing?
Here its only done with a gas analyzer, and i always passed that with standalones with better results than factory if tuned correctly for that.