you can use serial send block to send data over com port
you can use a constant block to give the input data of serial send block,just be sure to change the sample time from -1(inherieted) to sth else
to istrument block also should work , and many pages i read says it's working fine, but i tested without any result
about the string one , i never heared that u can send data as string
you can do as isaid,code your string as integer in transmiter side and encode it in the reciever side
you can use several blocks in simulink to do that