I did almost the same thing with you to the 29791 ICE 2 Starter Set. I bought from Marklin Service Store the current conducting couplers from the other ICE 2 that already have pre intalled from the factory internal lighting and replaced the originals. Then I changed the original decoder with Lokpilot 4 (not the M4 one for reason I will explain later) and I put a Lokpilot 3 FX decoder in the last non powered wagon. For sound I used a sound module (el chipo) that I spared from another convertion.
I also exchanged the pcbs for the front and rear lights so I now have white-red lights that change with the direction of travel. I added leds in the Cabs and in all of the wagons. So now:

Lokpilot 4 (with 21MTC adapter plate to have 4 AUX outputs)
F0 forward = Front White lights
F0 reverse = Front Red lights
Aux1 = Front cab (only operational when driving forward)
Aux2 = Sound 1 of Sound module (Horn)
Aux3 = Sound 2 of Sound Module (Closing doors)
Aux4 = Sound 3 of Sound Module (Conductors whistle)
Aux5 = Shunting
Aux6 = ACC / DEC off

Lokpilot 3 FX
F0 forward = Rear Red lights
F0 reverse = Rear white lights
Aux 1 = Rear Cab (only operational when driving in reverse)
Aux 2 + Aux 3 = Internal lights for all wagons (because the current was over 0,25mA I used in parallel to function outputs that are activated together)
Aux 4 = Free

I couldn't use the M4 version of the decoder because I have neigher a CS nor an ECOS. The mobile stations I & II that I use doesn't support consists. So I configured both decoders to the same address and they work just fine.

Because the couplers have only two contacts I couldn't use the feature the lokpilot have to change over the pickup shoe according to the direction of travel.

Thank you for your suggestion but...
I used for the internal lighting a LED strip of 4,8W/m, 60LED per meter. I bought 2 extra wagons so I had to light five wagons in total.
So 20cm of LED strip per wagon * 5 wagon = 1meter long. 4,8W / 12 Volts = 0,4A. I used a resistor in series to reduce a little the brightness of the LEDs so the currents went down to 0,3A > 0,25A of the output of the decoder. That is why I had to use to outputs in parallel. The next thing I could do is to use a relay and supply current directly from the tracks, but I don't like the "clicks" the relay do.

Anthpoa:

Any pictures of this conversion? Sounds very interesting.

Chris