Growing in a perfect environment for Haworthias (like Vista, CA, home of Steve Hammer, for example), it's actually pretty easy. The rest of your criteria are important, sure, but the weather is the main key, IMO, to getting large, naturally-looking plants as soon as possible.

I live in a similar climate to Vista, CA, which is not far from here. My climate is very similar to that of South Africa, and so South African plants do very well here without much care. I've not counted the heads on my Haworthia that I have in a pot, but it has quite a few. The ones I have in the front yard are a bit less prolific.

I get plants that are already adapted to my climate, and then generally they do very well in my yards. My soil is not great, but most African plants don't seem to mind, whereas South American and Mesoamerican plants usually need feeding or outdoor pots. My problem is that my soil is hard and does not drain well, but this is more of a problem for trees than anything else.