No, you've been perfectly clear, I got all that
Tasker can definitely do what you asked. It can turn on data for x amount of time, triggered by an SMS.
What I am saying is I am not sure that:
1) an MMS actually arrives as an SMS initially and if it does;
2) Whether turning data on
once arrived (regardless of method) would allow an automatic download to initiate
Tasker is a poweful and amazing app and it is definitely worth the purchase. However, if the above is the main reason to get it initially, I recommend doing the test I suggested (which is without Tasker by the way) to see if turning data on after receiving an SMS/MMS will allow teh data within teh MMS to automatically download or not.
I hope it makes sense what I am saying