Practically all main roads were sealed, the exceptions were usually only some roadwork areas. But the condition of the tarmac might vary (a lot!!) and sometimes you go through every imaginable road condition in 5 minutes....
And especially Iran, Pakistan, India, Indonesia have some very big speed bumps, that you cannot detect sometimes, and what´s even more dangerous, though it´s more rare, even the sewer hole lids might´ve gone missing (I dropped my front wheel into one of those in Pakistan, but luckily I was going very slow at the time – one of the good reasons not to ride in the dark!!)... so no matter which bike you choose, get some proper shelter underneath the engine. This is of course easier to find ready-made for dualsport-machines.
Indonesia probably had some of the most challenging roads, when ´main´ roads are concerned. There were some very steep hills, where the surface had practically come off with the rains (and they used Caterpillars to pull some heavier vehicles up!) Luckily those places weren´t too many. But still it was some 2500 kms across the islans, and the route we chose went in the mountains a lot (a good way to escape the intolerable heat BTW!) and it was mostly very twisty and slow... you could not go very fast, because drivers coming across might "borrow" you side of the road on the bends, traffic in general was almost as wild as in India, and often the braking zones just before the curves were, for some reason, extremely bumpy, and sometimes this was hard to see, because of trees casting shadows, etc. Just 300-400kms could feel like a VERY long day, I think you can do triple amount in Europe or in Oz, and not feel as tired in the evening!
But if you plan to go off the main roads, then you can find basically anything, and maybe a more off-road capable machine would have an advantage. But how much time have you got available – it´s a long route, and you WILL need lots of time to be able to do that a lot!?
If you´ll mostly stick to the main roads, then comfort on the highway is far more important in my opinion. But even the paved roads will often be bumpy, so the bike should be able to handle that. Also wheels that are made of cheese, might have a hard time, when you hit the bumps at full speed with a lot of load on board (and I can almost guarantee that it´ll just happen sometimes, even though you try to avoid that!)
To sum it up:
I think this particular route (and especially if your going to India via Turkey & Iran, and not around the north side of Afghanistan) generally had surprisingly good roads. India was the only place, where I expected them to be better (but every main road is paved there!) Also don´t forget your mileage, that you will do in Europe, and in Oz. I would not have a problem to do this route even on a streetbike. The big trailies are an excellent choice, because they handle the potholed roads much better, and still aren´t totally in the wrong place, even if you want to do the backroads a bit. The bike´s "real" off-road abilities will only be important, if you really are looking to go off the beaten track a lot.
(But this is only my 0.02, and you might hear very different opinions on the subject. I was riding 2-up on a DL650, and as you can imagine, this ain´t the perfect setup for very bad roads!)