Daniel - people don't care what you wear when you ride. If you want to ride in shorts and a t-shirt, then do it. If you find you get rubbed on your undercarriage then you may want to look at cycling specific shorts, eg knicks or shy pants.
really - if you want to spend lots of money on stuff go right ahead, but work out whether you actually need stuff, just want stuff or just want to look the part.
If you want to become a
competitive cyclist and spend many hours a day training, of course you will probably invest in some cycling clothes. it all depends on what you want to do.