Ultegra 600 STI Shifter compatibility questions

[SlowestPoke]

I have a Cannondale T700 Touring bike that I bought in 1994. I upgraded the components to Shimano Ultegra 600 with STI shifters. The rear derailleur is Ultegra 7/7 with an HG 90 8 Speed Cassette (13x26). I know this from the receipt that I still have. The front is a triple chain ring, but in 1994, the STI shifters did not do 3. So we found an aft4ermarket piece that you could add at the shifter to make it work 2 rings at a time (top 2 or bottom 2) by flipping the lever which lengthened and shortened the cable. After many years, my bike fell the other day and when I tried to catch it, I missed and grabbed the shifting cable. It broke the shifter where this added part connects and now when you try to flip the lever, it turns/spins at the connection point and wont work. Photo is attached. While I could replace the Ultegra 600 with a used one from EBay, they are pricey and I am considering whether it makes more sense to buy a triple shifter and make it easier to use all three rings. Problem is I am totally inexperienced in componentry don't have any idea what will fit/work, and I don't know what I need to know to figure that out. For example, can I buy a used Shimano Ultegra ST-6703 Triple STI Shifter off eBay and just put it on - or will I need new front derailleur, chain rings, what do I need to know.