The SE Midimini isn't much cheaper than a Model D in decent condition. The Source has a nice tone, but the interface is not at all to my liking (and it sounds like you don't like it much either). Neither has the sort of MIDI spec that integrates well into a modern production environment, but if you're into playing and tweaking by hand in realtime that might not be a big deal for you (unless the interface doesn't let you tweak much in realtime). I own a Voyager and I love it, having zero desire for a Model D, Source, or Prodigy (which you might consider looking into). The Voyager can be polite, but once you know its tricks, it can be an absolutely massive sounding synth. This is especially true if you combine it with an external oscillator and overdrive the mixer section. Alternatively, you can patch in an overdrive/distortion/fuzz/gain effect on the mixer-out/filter-in loop. But these "external" tricks aside, I've found that many of the complaints about the Voyager come from people not knowing how to program it. The whole "the filter doesn't open all the way!!!" complaint is totally false, for instance. For my money and preferences, the Voyager is the top gun Moog monosynth. Its synthesis capabilities far outshine the Model D, as do its modulation options [velocity, aftertouch, dedicated LFO, full ADSRs, tracking, XYZ pad, etc.}. It also has a vast range of CV I/O, stereo filters that can function in hipass mode, full MIDI implementation, and patch memory (if that's important to you). It was Bob's final word on the Minimoog. And you can buy it new with a warranty. I don't really care if it can sound exactly like a Model D. It can do things the Model D could only dream of. And it can also do a pretty convincing Model D impression, while it's at it.
If you really need to have _THAT_ Model D sound and don't really care about exploring the full depths of what analog monosynths have to offer, then by all means get a Model D or a MIDIMini. If that's what you want, that's what you want. (Also look at the SE-1X while you're at it, though). But if you want to go down the rabbit hole with a modern monosynth with a ton of modulation capabilities, synthesis possibilities, realtime control options, and more to offer you as a keyboard player, the Voyager wins. Especially if you learn to program it (and maybe throw in an overdrive or external oscillator whenever you want to whip out your Model D impression).