Hi s.i.v.a,
Chillispot would probably be the easiest way to achieve your goal.
Get a router that support DD-WRT with Chillispot
The web interface (admin/user) can be done by you or get an already done one (free or not).
Since most solutions are using MySQL, you'll have to do some hacking to get it working with PostgreSQL but thats about it.
The rest is only simple configurations/administration tasks that you'll have to learn any way to manage your hotspot.
Best,
Marcus