When it comes to creating an automated mailer you could have a script page with Universal Email applied to it. On this page you would need to have a recordset that selects all of the people you would like to send the message to. Then with Universal Email you would just set it to send to everyone in the recordset. To get the page to run every day at a certain time you would use a cron job on a Linux OS or a scheduled task on a Windows machine.
The second part about limiting the free membership is a little tricky. You must determine what criteria you will use to tell if the user has signed up before. The ip address is probably not the best choice as you could have more than one legitimate user with the same ip, also the ip address can be faked or altered very easily if someone wanted to gain access still. When you offer an opened free trial system like this it is very difficult to limit who joins the trial unless it is an invite only situation where you need an access code. Then you can limit who you give the codes out to. Post back with any questions you have and we will be glad to help point you in the rite direction.