Verified prospects are prospects that have had their email addresses discovered and verified by SalesIntelligent. You can add as many prospects as you'd like and only prospects that are verified count against your plan. For prospects whose email addresses can't be verified, you get the best-guess email address for the prospect for free.
All pricing is done on a monthly subscription basis for the number of prospects you need per month.
A plan can have unlimited users.
Yes, Salesforce integration is built-in. Prospects can be imported into Salesforce as leads or contacts.