Can I buy one custom DNS plus domain and use it for different host names?

Yes. If they all have the same Internet connection then you can either have one updated by your update client and set the others as CNAMEs of that one, or have them all updated by one update client. You will then have to forward a different port to each device, and remember to specify the port when you connect if it isn't the default for the service you're using.

You may find this guide to port forwarding and testing useful and if you need further help edit your question and let us know.

Edit

Thanks! But, how does "set the others as CNAMEs of that one" translate to the info I add to Dyn Standard DNS Service?

You've done that already for the www. record:

Host: http://www.domainwithcustomdns.com

TTL: 600

Type: CNAME

Data: http://domainwithcustomdns.com.

..is currently being used & returning errors from update clients.

Does that hostname have an IP address? If not you can't update it. Of course, if you were to publish your actual domain we may be able to help you more effectively.

I see the problem, you don't have any record for your domain itself (which I'll call example.org). You have the 3 CNAMEs set to example.org, but no records for example.org. Create one (a blank hostname) as an A record, with the IP address 127.0.0.127 (a dummy IP) and then configure your update client to update it.

Edit

That worked.. Thank you so much CH! Do I need to do anything with/for the other entries to allow me to use them on a few devices on separate networks: http://vpn.example.com http://nas.example.com http://pc1.example.com http://mac1.example.com

Yes - for each that you want to use on a different network you have to change it into an A record and have it updated by an update client. Just remember that you can only forward any port to a single device. For example, if you want to access the web interface on the NAS and a web server on the same network they'll need to use different ports - and you'll need to remember to specify the port when it isn't port 80.

Edit

Do I delete these cname records and "change them" to A records? http://vpn.example.com http://nas.example.com http://pc1.example.com http://mac1.example.com

Yes - delete the existing CNAME and then create an A record.