I've just been looking into this myself. I have a 30 yr old Moto, decided to leave it alone. If your rear hub cannot handle 9 speed cluster, which it certainly can't if it's an old classic, first you have to alter the rear fork (widen it) to accept a new wheelset with 9 spd cluster. STI only works with 9 or 10 speed. So you need new wheelsets (if you do the rear, you almost gotta do the front), new derailleurs, STI levers. Then there is the potential problem of your old crank, which may have the wrong spacing and pitch for the new chain that you need for the STI system. Depending on which level you go for (Sora, 105, Ultegra, DuraAce), it could cost a bit, including getting the frame opened up.