You could set it up as LAN installation, then you only need 1 external ip address, for testing this is good, but you need a seperate license too. You can't use the same license which you also have on your VPS. You probably know, but just to be sure.
I don't remember if DA can work on a dynamic ip, probably yes. But the problem will be DNS.
If you use an external DNS provider, you have to change the ip every time.
If you use local nameservers, you have to update your nameserver ip's every time with your registrar (this can take time to sync).
However, if you just want to test locally, maybe with pointing your pc's dns to your local server ip's, it might work for you.
DA will pick up your ISPs IP (guessing that's the variable one) and bind it to your DA licence in drirectadmin.com
No that was in the old days. It now binds your license ID to Directadmin, but it does check if it's used on multiple ip's regularly and then disabling DA if it detects it on one of the servers.
You do have the option to lock a license to your ip, but that is optional, not done by default.
I don't know what you want to test, but it might be better to use a local XAMP or LAMP setup.