The gear cables and casings you'll want to use are the common SIS spec ones that most every LBS has, and likely the only gear specific ones they have. Inner cable will be 1.2mm diameter and smooth surfaced (or redrawn/slick) made of either galvanized or stainless steels. The casing could be either 5mm or 4mm OD with the SIS spec uncompromisable wire winding. These casings require their own special end caps and are cut with a scissors action tool. The fitting of the casings and end caps to your older bike might be one challenge point. Proper cable routing and casing installing is but one aspect of a good shifting SIS system, but an easily screwed up one.

As to the compatibility of the front STI lever with your current crank and der only trying will tell you the answer. No charts or references are available (other then individual claims) that will describe the mix you're intending. Will the lever pull enough cable, or too much, to allow the ft der to move through the range that the crank's chain ring spacing requires is the real issue. Shimano has created a system in their index drivetrains and every component is matched with the others to achieve the performance they are known for. You could measure the rings' c-c dimension on the current cranks and then do the same with a 'current" 8 speed one and see if there's any difference. I suspect the old rings will be slightly farther apart then modern ones.

If you had campy Ergo levers things up front would be different as their lever goes through many steps (clicks) between the high and low positions. These levers can accommodate many different system parts and still work well. Andy.