I would say, if you want to drone you would need at least one sound source and one modulation. If you want to buy only one module, then I'd look for one that can do both simultaneously so as to modulate itself. You could get a complex oscillator: that's a single module made of at least two oscillators - and one of them would need to be able to get as slow as an LFO. For example, the Endorphin.es Furthrrr would be that. One of its oscillators goes into a wavefolder (which is a kind of waveshaper that makes mignificent purring drones) and the other can act as an LFO or a modulating VCO. That's the standard west-coast design and many other brands offer the equivalent: Verbos' Complex Oscillator, to name just one... Unfortunately, those modules are expensive (usually around $500) - they cost as much as two or three basic modules, which is essentially what they are, just bundled together and internally pre-patched.
Now you have other types of complex (read: multiple) oscillators than the west coast. More modern designs include Future Sound System's Recombination Engine, to name just one more - but they all have that high price tag in comon :-/
Just so you know my bias: I almost exclusively consider analog modules. My thinking is that for digital sound, I already have a computer. Not everybody agrees with me and that's fine. I've seen people do amazing things with all-digital racks. But now you see why my recommendation is different from @farkas 's one, which I respect nevertheless.
[EDIT: I now realize @farkas' recommendation is not that different: It is a complex oscillator after all. I had it mistaken for the same brand's quad wavetable)