The ST60 Tridata is about the cheapest way to get two important pieces of navigation gear into your kit - dept and speed. IIRC, the kit comes with the sensors, thruhulls, and instructions. You could go with dedicated instruments but I think the cost would be higher. FWIW, I'm planning on installing 2 Tridata displays in case one of them fails, but then I'm going offshore.
Your call, but you may want to consider a Multi instead of a second redundant Tridata. The Multi will not only duplicate the speed, water temp and depth data you get off the Tridata, but will also give you GPS, nav, pilot, wind, and voltage data. We've got a Tridata in the cockpit and a Multi at the nav station - the only complaint I have is that the Multi doesn't log max wind speed like it does with max/min depth.