[Ticket system support] Ticket system support

Hi,
Actually, i am searching for a system to do an aim. Let me explain so you can tell me may be you have add-ons that can help me.
First, we are a web agency and we offer wordpress support and maintenance to our clients.
Our goals are :
1) be able to recognize our maintenance clients (and differentiate them from others) that open tickets using email piping .
2) we want to sell packages of hours of maintenance online to clients and non clients, (clients need to have a discount.)
Let’s say 1 hour, 5 hours, 10 hours…
A client or a new client choose the package and from this moment he is automaticall assigned to a support staff user and he can send support requests.
The goal is that support staff should know/understand if its a new client or an existing client and if it’s a new client if this one has paid
So just to better understand that point
Is there any solution for that ??
Thanks a lot