upgrade from MF-TZ21 to 7 Speed cassette freehub
Pretty sure that the MF series has the freewheel integrated with the cassette, so if you are going to a "pure" cassette, you will need to replace the freewheel and possibly the hub.  You would need to replace the hub if you can't find a conversion to a non-integrated freewheel for your current hub.
If the hub gets replaced, that could affect the disk as well.  Does your current disk have an ISO 6 bolt pattern, or something else like Shimano's centerlock?  If it's an ISO 6 bolt, you should not need a new disk with a new hub, assuming you get a hub with an ISO 6 bolt pattern.