I recommend monitoring the connection to the Internet router, not every switch. If you monitor all the switches you will see a lot of LAN only traffic which has the effect of hiding the LAN to Internet traffic, the stuff that cause recurring access charges. NTA can use NetFlow v5 and 9 as well as Sflow v5 and J flow. Take a look at this document, especially the Deployment Stratigies section.
Unless this has changed recently, keep in mind that the license for NTA will need to match the license for NPM. That is, if you NPM SLX, you will be required to purchase NTA SLX.
I'm a little unclear about your requirement to monitor all workstations. Unless you're going to be monitoring such information like CPU, memory and disc space utilization, you won't need to need to install SNMP on them. You definitely don't need SNMP for NetFlow. The NetFlow exports from your internet router/switches should provide all the information that you need.