I have an extensive collection of older (15-20 years) Z-Scale Maerklin equipment.
Can I convert the locos to Digitrax, or must I buy some other brand of loco to use Digitrax?
Maybe yes, maybe no.
It depends upon the size of the locomotive.
We are aware of several European Z-Scale model railroads that use Digitrax.
We also know of a very large Z-Scale railroad in Washington, DC that uses Digitrax.
This particular railroad is a model of Swiss prototype, so the locomotives are all electric-types; there seems to be enough room inside for a decoder, especially the new DZ125 series (DZ125, DZ125IP and DZ125IN).
At the present, the smaller Maerklin locomotives, such as the 8800-type (Class 89 0-6-0), the 8864-type (V60 Class) and the 8895-type (2-6-0T engines) are too small for any current decoder.
The Class 24 2-6-0 gets a "maybe" rating because it may be possible to install a decoder in the locomotive's tender.
The larger steam engines also are "maybe's" for the same reason.
The four axle diesels are functionally similar to the electric locomotive's mechanism.
The only electric that should be ruled out is the Swiss Crocodile; if you've ever taken one apart, you'll never do it again since reassembling them is a hairy experience.
Maerklin mini-club motors have traditionally been 8 Volt DC motors, so you would need to adjust the output voltage of the decoder, probably adjusting CV05 to a much lower speed setting (thus a lower top voltage).
Of course, Digitrax has a long standing relationship with Micro-Trains and our DZ123M0 is designed to be installed in their GP-35.
As an aside, when Maerklin first introduced their Digital command control system in 1985, the Company stated that Digital would be available for Z-Scale in a few years.
A few years stretched out to many years, and the Company finally admitted that it was not going to happen.
The reason given at that time was that they could not find wires which were flexible enough to be connected between the locomotive and a decoder carrying car (such as a tender).
With the fullness of time, we now know that the problem was more likely the problem of heat build-up which was present with that generation of decoders.
If you do convert your locomotives to Digitrax DCC, we'd love to know about your work.