Having just rebuilt an old Healthways Scuba, I don't think I've ever seen anything simpler. I understand the original Aqualung Mistral is similarly simple though and parts are more widely available.
I'm not really sure how important being bulletproof really is in a regulator however. There are lots of regulators out there that will hold up for years of diving without much more than a rinse off after use. Old single hoses are typically dime a dozen too, so what I'd do is pick up a couple of old MK5's or Conshelfs as you find them for cheap and then use one as a backup. Then pickup a third when someone offers you one their uncle used to dive with, or whatever the story is. I paid $10 for a Sherwood Brut, $20 for a MK5, was given another MK5 and a MK7, got a Waterlung as part of a $50 package deal along with a set of old doubles and got a MR12 as part of another package. Typically, I inspect the diaphragms, inspect the hoses, make sure there are no free flows or IP creep and that they breathe OK and then take 'em out and dive 'em.
Some of those old regs don't breathe all that spectacularly, but others like the MK5/R109's breathe just about as well as my MK25/G250V. The real issue is whether they were performance regulators to begin with.
