Captive portal with internet access to user

I have Raspberry PI acting as WiFi hotspot. It has a local web site hosted using nginx and tomcat. I'm using this setup at a garage and want customers to use this for local service to stream audio and some garage videos hosted using web page. These things I need to implement on the hotspot: 1. When user connect to WiFi using phone the local site should auto launch in his mobile browser. 2. There is also internet available using 3G dongle. I want user to access internet and system should be able to manage how much time\data user can use based on some configuration parameters. Also I want user to access specific sites only. User should be able to use the local site when access to internet is not available or available.