We are planning to buy 60 AP-105 for wifi. We can manage the AP's with one master AP. - OK, you're talking about Instants? Yes.
But we can also buy a Controller or another instance like airwave. - You could do.
Plannig is : 2 networks both a own Radius. - Ok.
max 1800 users on the wifi network. - Sounds managable (15 clients per radio).
What is the disadvantege of making one AP the master and configure everything on that AP. - You are talking about instants, and the virtual controller? There aren't any big disadvantages in this case.
Are you thinking Instant vs. controller? If so, the biggest difference that jumps out, is that you might have to tag the WiFi VLANs around all switch ports to which instants attach (depending on your logical design). With a controller, it only need be at the controller ports (assuming a L2 design).