Absolutely agree. Typically I deploy about 4-6 "tools" hosts in a site. From a computational standpoint, you could make do with fewer hosts, but there are some things that I prefer to separate out:

1&2. Authoritative DNS (internal)

1&2. NTP

1&2. Caching DNS resolver

3&4. Outbound HTTP proxy (if necessary)

3&4. PXE / installer / dhcp

3&4. Local software mirror (apt / yum / etc.)

5&6. SSH jump hosts.

Or something like that.

Make sure the second host is not just in a separate chassis from the first host, but also in a separate rack.

For external authoritative DNS, don't do it yourself -- pay for someone to run that for you (Route53, ns1, etc.). For e-mail, if you can possibly not deal with it then don't -- use Mailchimp or something.