1) If it is better to enter the DNS server addresses manually?
2) Is it better to use Google DNS instead of the Orcon IP's? Ie is it faster overall or less likely to Ddos attacks?
I'm not sure what posts you're referring to - a default password has nothing to do with DNS.
Many older routers are insecure and allow recursive DNS lookups from the WAN meaning your connection can be used as part of a DNS amplification attack. This has nothing do with passwords, it's either routers that don't have firewall settings allowing this to be disabled, or routers where people purposely disable it. I've seen a couple of posts over the years here on GZ from people who have purposely disabled all firewall settings and don't understand why this is bad.
You should ALWAYS use your ISP's DNS servers unless you understand why using 3rd party DNS will result in a degraded Internet experience. There are situations where 3rd party DNS is handy, but for 99% of people it should be avoided.
Your choice of DNS server has nothing do with with DDoS or DNS amplification attacks which are totally different.