Just skimmed through this thread, so hope I don't duplicate anyone's post!
I've been using a Vivente World Randonneur tourer for 3.75 years now, and have almost done 50,000km on it. It did need some tweaking, as they all do: handlebar angle & height, seat position, etc., but I find it comfortable enough. I can't claim to be a real Audax-style cyclist, though, as my longest ride is still only 250km, with no multi-day Audax rides.
I use a 50-39-30 triple on the front and an 11-34 cluster on the rear, so enough gears for all occasions. My tyres are Schwalbe Marathon Plus 700x28C always pumped up to about 100psi. I use SPD pedals, as SPD-style cleats are easier to walk on (although I don't try to walk long distances in them).
I also use a Son-20R dynamo hub with Schmidt Edeluxe front and (I forget the brand) rear light, backed up with some USB-rechargeable lights. My brakes are cantilever rim brakes, which is sometimes a nuisance, as the continual hill-climbing I do (roughly 5.5 to 6km vertical / week) wears out the brake pads (and wheel rims) way too fast. My previous rims (just replaced a week ago) only lasted 6 months.
Not trying to sell anyone on the above combination, by the way, just offering one of the options.