Is MSG necessary or what to replace it with?
You should refer to the discussion here:
Are MSG and Accent (Seasoning) the same thing?

Some relevant points:
MSG is found naturally in seaweed and other things. It is used to enhance the flavors it is combined with.

It is completely harmless unless you are limiting your sodium intake. Check out the wikipedia article on the subject as a starting point:
http://en.wikipedia.org/wiki/Monosodium_glutamate#Health_concerns

In short there has been no linkage to any adverse effects found in many controlled studies.

As far as replacing it- You can leave it out altogether or add a little salt.