Hi,
The better [read more flexible] solution is to get Symantec System Recovery Desktop Edition. It will do the things you want done. Make a new full backup each month, do incrementals in between and when you have the number of backups you specify it will make one more before deleting the oldest one.
It is also able to do lesser backups of different material and store the backup on a different drive.
Take a look, It is a licensed procuts so there isn't an annual fee to retain it.
Keep us posted
