Congratulations on your new assignment! They must think a lot of you to give you such a big responsibility.
As for your questions, I look at a web service API just like a conventional web application. The only difference is your clients will most likely be code than people. So all the same web application scalability principles you already know--load balancing of your web servers, building index and views on your database, caching--apply just the same here. You shouldn't think of this any differently. So just the same, make sure performance tests with JMeter or a commercial product are a part of your continuous integration infrastructure.
As for supporting different media types, I don't know exactly what you mean. Since you mentioned Spring, Spring and Spring Web Services will cover everything you need to get going fast. There are numerous API-level things like authentication, authorization, logging, auditing, error handling, etc. you will need to address. There are a lot of resources out there to answer your specific questions.
One thing I can also say. Please be careful about coupling. Your WSDL's and REST API's will be sure to evolve a great deal. Make sure you code in such a way that those changes don't impact the rest of your code base. Or you will be working a lot of weekends to fix everything after a small change.
Good luck with your project!