Look at this sites:

http://www.hr/index.en.shtml
http://www.adriatica.net/croatia/main_en.htm

Quality of roads?

From Dubrovnik to Zadar - excellent
(many bicyclist ride from Dubrovnik to Rijeka, probably
preparing for some tour...)

From Zadar to Rijeka 8/10, but still very good.

From Zadar to Zagreb, 5/10 - there is havy traffic and in some
sections road is
very curvy and there are climbs.
distances (cca)
Dubrovnik - Split = 280 km
Split - Zadar = 200 km
Zadar - Rijeka = 280 km
Zadar - Zagreb = 280 km

There are hostels in Zadar, Zagreb, Split, Dubrovnik, Rijeka
so you want have problems with that.
There you'll find information about Croatia (general) and
you can search for a information you need.

If you are interested in some more information, just ask.

Branimir