Hi Matison
A warm welcome to our forum
Here a French fan of Märklin digital HO with computer control (Rocrail).
Märklin HO Digital is an excellent choice. I use to run N Gauge in analog and indeed some parts are really tiny (better have an eagle's vision
Your conversions
1. 3324 NS loco 1983-1987 Probably with a Drum collector (to be checked). Motor to be replaced with Mäklin 60941. Decoder mLD3 to be added. Difficulty: medium
2. 33292 Loco around 1996 E91 of the DRG Probably with a Drum collector (to be checked). Motor to be replaced with Mäklin 60941. Decoder mLD3 to be added. Difficulty: medium
3. 37184 Loco BR 18.4 of the DRG 2000-2003 Loco is already digital with simplyfied sound (runniçng sound, bell, whisle, smoke). Nothing to do unless decoder is damaged
4. 37610 Loco PA-1 (ALCO) of the U.P. 2002-005. Loco is already digital with (simplified sound- operation sound in sync with speed). Nothing to do unless decoder is damaged

Märklin is really a top choice and digital operation is fantastic.
You may use a Mobile Station 2 (60657) with a track box 60116 and a power supply adapted to your country
Alternately, you may use a CS3 which is more expensive but much more powerful with huge possibilities
I strongly recommend you use C-Track but only purchased new. Early C track had a plastic formula issue and became very brittle.

Here is a small glimpse of Märklin digital operation (my layout)

Enjoy your trains
Jean

As always, Jean's response is fantastic. I have that old C track and broken pieces everywhere. I much prefer the look of K track over plastic look of C, especially with track bed. There is K to C connect track, so you can mix their use (like C in hidden areas).