I'm calling BS on the "doctor ordered a 24VDC CPAP" line from the DME providers. The Docs I've dealt with order a CPAP or an APAP as they see appropriate, and the only specifics on their order is ... "set at xx cm WC" or "set at range of xx to xx cm WC". The brand, type, voltage, mask, etc are all pretty much open to negation with the DME provider, what they carry and the willingness of the respiratory therapist to spend time with you.
Since our first encounter with them is almost always met with full ignorance on our part, we take what they have without argument, especially when they throw in the "doctor's orders" **** to enforce their point. After self-education, one can find a number of good outside resources on discussion forums, *PAP groups, and on-line Respiratory Care vendors to assist in self-managing their own care.