CMS & WordPress
Suggest Me Open Source Content management system, feature with
Unlimited Page/Form Feedback/Menu
Permissions to limit access to certain users
Easy to add custom webdesign templates.
easy to create/edit/update/delete content
Search Engine Friendly URLs with mod_rewrite
and must be not able to find script platform by any cms detector.
Did you look in this thread? That lists a number of the packages available and breaks it down by programming language, so you can find one that will run on your server.
Explore them (a large number have demo sites available). Find one that has the features you like and go with that one.
I would suggest what Dave mentioned above however your last criteria is going to be troublesome... Anyone familiar with a CMS will be able to figure out what you're using based on the file system structure. If you want it to look like a custom CMS, you're going to have to build a custom CMS. Otherwise I'm sure www.BuiltWith.com will figure out what you're using.