nice trip you like to do. Well, the quality of the roads depend on the type of road you choose. For example in Peru, the main roads are usually quite good qualitiy. We once did the distance from La Paz to Cuszco in one day (~700km). But we took the road south of the lake Titicaca, which is all asphalt. Also the raod from Cuszo to Arequipa via Juliaca is complitely made of asphalt (Av. 80-100km/h possible), even if it is not written in the map. But the direct road to Arequipa is a dirt road where you have an average speed of maybe 30-40km/h. So you are faster to take the longer way. And it is a nice route too. Also in Chile and Argentina the main roads are of good qualitiy. However, as soon as you take smaller roads, which I recommend to do, you will find a lot of gravel roads. No problem to ride, but slower (for example Paso Sico, Paso del agua negra, or visit of places aroud San Pedro de Atacama like springs of Puritama...).
In Bolivia the main roads near cities as well as the main connetion roads between big cities (like La Paz - Cochabamba or similar) are asphalt. The rest is more or less made of gravel or dirt roads. The main roads far from cities are mainly good quality gravel roads (aver. speed appr. 70km/h possible). The little roads to little villages can be in a very bad condition so that you might not be able to pass. Also there are just few bridges, so if there is a lot of rain you might not be able to pass rivers.
In my point of view, most people (andes-bikers) leave Brasil out of their planing, even if there are a lot of nice places to visit.
Anyway, have a nice ride and enjoy your trip