I'm sure you will get lots of recommendations, but you can also search for "interchangeable needles" using KH's search feature. You will find lots and lots of threads about this very subject and all kinds of opinions on which needles to buy (or not buy). You might check those out as part of your research into the world of interchangeables.
By the way, I have one set of interchangeables (Boye Needlemaster), which I love, and then I have some plain circulars (well, they're wood and bamboo), DPNs in various sizes, and inherited and bought regular needles. So I'm not so sure that the mere act of buying a set of interchangeable needles necessarily translates into spending less money!