Captive Portal Setup

We run a long-term guest house that provides wifi to our guests, and we require a captive portal solution. We currently have 4 Unifi access points and a DSL router. We wish to put a Linux server between the wifi clients and the DSL router to provide a captive portal. The captive portal system should allow: Creation of user accounts, that are allocated 5gb per month free of charge (this renews monthly, but should not accrue... so if the user only uses 1gb one month, the next month they should reset at 5gb). If a user runs out of the 5GB allowance in a month, it should take them to a page requesting them to buy an additional data pack: 1GB for $5, valid 30 days 5GB for $10, valid 30 days 10GB for $20, valid 30 days 20GB for $30, valid 30 days 50GB for $50, valid 60 days 100GB for $100, valid 90 days Pricing is in New Zealand Dollars (NZD), and should direct the user to PayPal to complete the transaction. Portal should verify the payment with PayPal prior to crediting the user's account. We wish to be able to view traffic stats on a per user basis as well. We will provide login details for a clean, new installation of Ubuntu Linux on a physical server. We can configure internal IP addressing according to your specifications.