200 is the local storage. As long as you've got an internet connection you should be able to look at older emails. Just scroll to the bottom of the mail list and choose to load more emails.
I don't know of the search feature will pull in emails from the server with an Exchange account. I recommend experimenting to see if it will. Combine it with the load more feature. My only Exchange account (GMail) has far too little traffic for me to test it myself.
There are also some apps that will read .pst files. Check out the App Store. You could periodically export and import your important mail folders to one of these apps on the iPad. I don't do this, so I can't recommend a specific app or method.