This is ungrateful question, because there are many impact parameters. In this case (I guess your database will be a small - up to 20-40GB), by my experience (minimum):
-Processors: 2 x 3GHz 2MB L3 Cache
-RAM: 8-16GB
-Internal Storage or Direct Attached Storage: 4-6 HDD+ Mirrored Data Disks and 2 HDD Mirrored TransactionLog Disks.
For 10 users, you can place NAV server and SQL server on the same server, but for 30 users, better solution is to place these servers on different servers. I don't know what you else want of information.