billion, netgear, d-link are pop brands in SA market, any of them just works fine with telkom line
adsl router/modem is a modem integrated with router and other network functionalities (NAT, firewall, VPN etc), those functions will make your adsl line more easier to maintain, more securer and valuable
one simple application is: sharing your ADSL line for more than one PCs, it's simple as just as plug and go.