=> I became interested in the Marklin Digital brand, because it =>enables you to control the whole system more conveniently.
Well, yes it does, but it does not conform to the standard DCC (Digital Command Control) specifications used by everybody else. Marklin has a history of refusing to go along with standards, which means in practice that it's usually some kind of hassle to run Marklin equipment on anything other than a Marklin layout, and vice versa. Since Marklin, despite its size, cannot produce a model of everything you might want, that can crimp your style. (BTW, Marklin could have adopted the DCC standards, but then its customers would be able to buy someone else's product to run their trains, thus depriving Marklin of its rightful profits....)
As someone else has suggested, check out ordinary DCC. It's cheaper than Marklin's stuff, and getting cheaper all the time -- because it's all built to the same standard, is interchangeable, and will run anything.
Having said that, there are downsides:
A) You do in fact need three types of components, whatever system you opt for: a) the Command Centre/Power source; b) the Controllers, and c) the Decoders. You may also have to isolate a short piece of track to program the engines. I suggest you try to find people within easy driving distance who are using DCC, and ask if you can come over to see how it works, and to get advice. (You will also make contact with likeminded people, and probably make lifelong friends - a bonus!)
B) Installing decoders in older equipment may be a major hassle and may even be impossible. You may have to mill away part of the frame or chassis to make room for the decoder, build your own connector harness, and so forth. Again, in-person advice from someone who's been there is essential IMO.
C) Converting an existing roster will be costly, and may limit your operations. However, if you break your layout up into "operating districts" ("super blocks," if you will), you can continue to use conventional control on some parts of the layout and DCC on other parts during the conversion phase. Just limit different locomotives to different parts of the layout - a very prototypical thing to do anyhow. You may need a little new wiring to accomplish this. The operating districts can stay in place when you have fully converted to DCC.
Re: computer control and DCC. Using a computer to do what DCC does has no advantages - you still need decoders in every piece of equipment that you wish to control from the computer, and having just _one_ controller (the computer) for _all_ your engines vs having one controller for each engine is not exactly an improvement. Computers have their place if you wish to automate the whole layout - but that's whole different task, one that requires not only electronic but consderable programming expertise. BTW, DCC developed out of several pioneering attempts to use computers exactly as you suggest - it's in effect a dedicated computer system.
Computers are excellent for generating timetables or other aids to operations, and for simulating or enabling dispatcher control of a layout. Neither of these has any effect on how you control the locomotives, however.