Thirty years ago, road and touring bikes were not that different from each other. However, over the years they have each become far more specific. You can still take a roadbike, put some clothes in a saddle bag, and ride from one youth hostel/hotel to the next. That, however, is about it. Also, you will be very restricted in where you can ride. Bad roads will take a great deal of finesse, and gravel roads are completely off limits.
This, on the other hand, is where modern touring bikes shine. The main roads have become so much busier, that touring cyclists have escaped to the minor roads, and sometimes even gravel roads and the like. Riding there has also become easier with the avaibility of mtb parts and gearing for touring bikes. Thus, the emergence of the modern semi off road tourer: the 26 inch bike with clearance for wide tyres, wide range gearing, and traditional drop bars. This is what I ride, and I think it is perfect for modern touring conditions. Fit 50 mm Schwalbe Marathons, or even Marathon XR's, and there are few places where you cannot venture. Fit 26x1.75 Panaracer Pasela's, and you have a pretty fast and nippy bike (but not a racing bike).
The audax bike is somewhere in between, but yet it is not really a jack of all trades. It is not as fast and precisely handling as a competition road bike, although that may not matter too much. More importantly, and depending on your own weight, it cannot handle that much luggage. If you yourself are light, and if you choose 135mm hubs with 36 spokes, ultralight camping gear (12-15 kilo) should just about work out. However, this will still not let you ride a bit off road, and thus restrict where you can go. For that, a minimum of 35-37 mm tyres is required, and that means cantilever brakes.
So, if you want a jack of all trades, I think it has to be a fastish bike that is up to loaded touring on bad roads. The Dawes Galaxy is a classic choice, but so is the Thorn Club Tour. If you want to spend more, any good framebuilder can build you something like this, including variants with a Rohloff hub.
Fit wide sturdy tyres, and you are all set for touring. Remove rack and mudguards, and fit 30mm Grandbois Cypres tyres (or less extremely 32 mm Panaracer Pasela's) and you have a reasonably fast bike.
Alternatively, you can go my route, i.e. get a really nice 26 inch tourer, and spend a little bit on an old secondhand racing bike with clearance for mudguards and 32 mm tyres. Old racing bikes really are just audax bikes by modern standards. And they can be had for next to nothing. A modern audax bike is obviously nicer, and that is what I am saving for at the moment. But it is really not necessary if you want to have fun.
What I am missing on my old racer, however, is the Schmidt generator hub that I have on my touring bike. A winter training bike really should have one. That could be a reason for avoiding an older 27 inch racing bike such as I now have (not much point in still getting an expensive wheel made for it).
Willem