Is it a idea to make a mod in de admin-panel wich checks if folders and files are in the right CHMOD status?

This will be a nice extra in the admin-panel.

It's a nice idea

Make sure you don't use to many chmod 777 and use 755 instead, but it's up to you.

This is a very nice idea as even me I forget to change the perms to a more strict policy. Like I said above, try not to use to many 777 (world readble) and use a more strict policy of 755 the 777 gives more places to hide exploits and other mischief. Very good suggestion though.

I'm building a diagnostic page in admin, not only chmod

I would imagine you would, and it will look very nice.

That's great MEM, i hope it will come verry soon.