Hello Jorg
Thank you for joining our community of Märklin fans.
Here a faraway Märklin digital HO fan from Paris France with a liking for automation and computer control (Rocrail).
Being a long time collector, the only Märklin HO track I don't have is the prewar track so I can compare the types easily.

M Track:
- this is an excellent track especially when in good condition and of the later manufacturing (1980-2000).
- very noisy compared with others, no long contact strech rails and early points have large lanterns incompatible with long rail vehicles

K Track
- very modern when it appeared in thge late 60s, less noisy of all
- by todays standards it is very crude with enormous rails, studs
- until recently the most developped offering of Märklin
- when ballasting with glue, often develops contact problems (so DO NOT)


C Track
- by far the latest Märklin design (first design in 1996 but improved many times since
- excellent contact between rails , excellent mechanical joining of rails, the best in class
- a bed to host motors and decoders in a perfect way
- the track is being completed with new items (sleek DSS, turntable, etc)
- to reach perfection, you need to paint the rail sides in brown
- like with K track, it is very easy to make long detection streches for maximum security

I strongly advise to use C-track
I look forward to see your projects and exchange on train topics
Here is a video of my permanent, mostly C track Rocrail-operated layout
https://www.youtube.com/watch?v=F6WnhXPcRqI

Cheers
Jean

K-track is very nice.
The K-track rail-profile is around code 100 (106 to be exact), which is a very common size of track today.
Newer K-track (post year 2000) uses electrical connector made from phosphorized copper, which doesnt easily corrode. The studs are made from iron, -same as C-track and M-track- and is prone to corossion.
The huge advantage of K-track is its slim geometry which allows complex trackage in much less space than with C-track.
Another huge plus is the existence of flexible tracks, which the C-track system tragically lack!
K-track should be ballasted, which makes for a superiopr look compared to any C-track layout I have seen. This is NOT a problem (even with older K-track pieces), as long as you use acid-free glue and an alcohol-based wetting agent. (I use 50% destilled water + 50% cleaning-alcohol)
If you are looking for making a permanent realistic-looking beautiful layout, you can stay with K-track. Maybe you can use your old M-track in hidden areas.
The special transition-track between M and K track is not necessary; The rail code is the same, and you just modify the K-tracks with a Dremel, and push them together...
K-track doesnt take well to continuous rebuild, so if a permanent decorated layout is not your objective, you should probably go the C-track way.
C-track is has a slightly finer profile (Code 90). Personally I dont find the difference important; There are so many other things much worse in the looks of C-track. (like the ballast, which makes it look like M-track)


If digital is the priority for you, you should probably start from scratch with a new MS2 starter set. Get rid of all your old M and K track (Ebay¿?)
Even if older Delta-locos tecnically will run in a modern digital system, they are really not up to the level of the modern all-sound+light+smoke digital machines. Check the offerings from other suppliers like ESU, what they have of dynamic smoke and light+sound effects...
-so you would minimum need to fully digitalize any analogue loco, and probably want to change the decoder of all Delta machines.
Depending on the type and age of your Deltas, you are probably also looking at a full high-efficiency motor-update on those too!