One of the best things about the arduino platform is that there are a lot of boards out there (called arduino shields) that you can simply stack on top of the arduino board headers -- no wiring is required! Just try googling arduino shields and you'll see. I've found that building something you would personally use keeps you motivated and excited about learning new things, so choose a project you would enjoy to help you figure out what you want to buy first.
You'll find that a lot of modern sensors and ics make use of spi and i2c and i would recommend that you learn how to make use of these technologies as they are usually integrated solutions and speed up the development process.
If you want to develop your skills for prototyping, i would also suggest you learn how to etch/prototype your own pcbs. I have personally found them more reliable and robust than simply using the breadboard or universal prototyping pcbs :thumbsup: