I am not certain what you mean by "digital fax" but if you mean the ability to fax a document right from your computer, without having to perform the ridiculous step of printing a document just so you can fax it, then you should consider Brother's line of multifunction devices.
The one I use predates AirPrint but they now have a number of printers with that feature, and also fax directly from a standard Print dialog box. Brother supports OS X very well with its native CUPS interface, they are inexpensive, their ink (in the case of inkjets) is inexpensive and lasts a long time, their photo quality is at least as good as HP's, they have lifetime free technical support with people who understand OS X, and a two year warranty.
Read the reviews (Amazon for instance, but be sure to read HP's and Brother's user reviews on their respective websites as well) and decide for yourself.
Brother™ Monochrome Laser Printer and All-in-One models now support AirPrint.
Brother™ Color Inkjet All-in-One units now support AirPrint.
I have no affiliation with them but I would not want a Mac user to make a purchase decision they'll regret 😉