Almost any old loco can be converted to digital, the ones you mention are a "breeze" since there are conversion sets available from Märklin for all these... However, there is a cost associated with the sets and there are other ways to rebuild the motor and use cheaper decoders.

Regarding track, if you are going to build a separate track with new track pieces C would be the best option. However, there is nothing wrong with eg M-track in digital if you happen to have a lot of them.

If you want to make a "fresh start", a digital starter set with a controller, digital train(s) and C-track is more viable regarding getting into digital from a finacial point of view if you want to get the "feel" of it in a quick way.

A Start Set is a useful way to enter the digital world. There is a reduction compared to the individual items. If you cannot find a start set that interests you, then you may wish to buy the elements separately. You can also likely find broken up start set elements on eBay and at some retailers.
If you intend to operate it alongside your existing M Track layout, you can consider the larger radii 4 and 5, which would allow for you to set up a mainline arond your existing setup. My plan was to do something along these lines, with the larger radii representing a (potentially high speed) main line, with trains like the TGV, ICN, IC2000, etc and have the interior tracks used for local and short trains. You could still set up a common bahnhof for the two sets of tracks as the focal point of your new layout.

Regards

Mike C