I'm not sure if you are aware of this but I will put it out there just in case you didn't know, even though the M4s and MP5s use version 2 mechboxes not all of their parts are the same.
Would I be correct to assume that this is the kit you are purchasing?
http://www.airsoftparts.ca/store2/in...oducts_id=1055
If it is you will need to either purchase a new selector plate for your mechbox or transfer the existing selector from your old mechbox to your new one. I believe you may also have to replace the nozzle as well.
However I did notice that airsoft parts did have an MP5 drop in box available as well, a quick spring change and you are good to go.
Edit: Please see the post below for a complete list of parts that will not be compatible.