Convert a Cannondale SM1000 (about 1991 model) to disk brakes
Adding a front disk brake:


Possible?  Yes!
Financially feasible?  Probably not.
Worth doing?  Only as a learning experience - don't expect it to turn out as good as a modern bike.


Your bike is 25 years old, and as such it will be a product of its time.  The fork will not have mountings for disk calipers, so you're up for welding some on, or replacing the fork.

If you weld, hopefully its a rigid fork because welding anything onto a suspension fork is incredibly hard, plus the risk of damaging the suspension.

Replacing the fork is easier, but you're going to need a 1" threaded MTB fork with disk mounts.

If all the other parts swap over unmodified, you're very lucky.

Do you have any emotional attachment to the old canoodle ?  Modifying it can be a bad idea and spoil your connection with the old bike.

I suggest you price up a new fork, wheel, caliper (both cable and hydraulic) rotor and see what the total parts cost is for both sorts.  Then go try some new bikes in a real shop, and calculate the difference in price.   You can even subtract whatever you'd get for selling the old bike, or revel in becoming a cyclist whose N value is now 2.

Just remember rule 12 (http://www.velominati.com/the-rules/#12)

If you can get the parts for nothing, then by all means its a good project.  However it'll never be modern - instead its mutton dressed as lamb.