Lots of good info here:
https://netsuite.custhelp.com/app/an…k%20permission
As for a real answer though, it depends. You will absolutely have to have Permissions > Setup > Web Services, but depending on what you want to do with that integration will depend on whatever other permissions you give. If it is to just pull some sales orders, you will need Permissions > Transactions > (insert transaction type here), etc. You need them to define what the integration is going to do, then look for those permissions and apply accordingly.
Run into a permission and you’re not sure what it is called? Use this nifty trick:
https://netsuite.custhelp.com/app/an…k%20permission