How can IPv6 clients find servers such as NTP, SIP

DNS can be taken care of by router advertisements, but as for all other options, you're basically stuck with DHCP. However, a stateless DHCPv6 server is one valid deployment, where the DHCPv6 server doesn't track IP assignments, but only advertises services to hosts in the network, and let's SLAAC configure IP addresses.

Options seem to pretty much identical: http://www.networksorcery.com/enp/protocol/dhcpv6.htm

RA seems to only allow you to send basic IP information and the default gateway, DNS might be possible but nothing more.

So TL;DR;

Your options are to either use stateful DHCPv6, which works exactly as DHCPv4, or you can use stateless DHCPv6, which uses SLAAC to do IP configuration, and as such is much more lightweight.