You can create as many contracts you want for your clients and they can see all the contracts and their history in their portal. They can easily sign and download the contracts.
Your clients can see all the projects you have with them. They can share files and see the progress of the projects. You can even allow them to see tasks if you want.
If your clients need any help, they can create support tickets from their portal to get immediate help. They can see all the tickets they have created and their status.
Forget cluttered emails. If your clients wants to talk to you they can send you a message from their portal and you can reply them to keep the conversation in one place.