This should be possible as long as your current platform provides IMAP access. This would be a billable service based on the amount of time required to complete the migration. We would confirm that the migration is possible prior to any support billing.
Yes. Just reach out to our support team after purchasing the Google Workspace product from us (one for each user) and we can complete the migration.
You can have an unlimited number of users. Note that each user requires a Google Workspace product purchase.