Validation In Database Through Statement Level Trigger

I have a table which contains data of all calcualted payements for a Land for a particular period i.e. for a Land 124 for the duration of Jan-2010 to Mar-2010 100 rupees rent is paid to the owner.Requirement here is that for a particular duration i.e is for Jan-2010 to Mar-2010 system calculation of rent for a land file multiple times where the table would contain history of each transaction i.e. a user can calculate for a Land 124 for duration fo Jan-2010 to Mar-2010 rent more than ten times and there would be ten records in the table but only one record out of these would be valid where the user cannot calcualte rent for Lnd 124 for Jan-2010 to Mar-2010 without updating the status of the last record in the table for the same land and duration to invlaid. I want to add this business validation through triggers as i am not able to do so through check constarinst.I want to ensure that only one record should be valid for a particular land for a particular duration since row level triggers do not allow us to query the table ion which the trigger is executing i am trying to use statement level trigger but i have a few doubts here: Incase of a Before Insert statement level trigger does the trigger have access to the data which is being inserted into that the table cause whenever a record is being inserted into a table the systen would check through the trigger that if there is a valid (payment_status is valid) payment record in the table for that same land file and same duration.If there is a valid record then the trigger would through an exception. I know you can use a row level trigger with an autonomous transaction should work wihtout giving the mutating table error issue but i wont to know if using a statement level trigger here is feasible!!