igorpan,

Roland E-50 cannot create new sounds (samples to be precise) but can change existing ones (because sound is sample + various, changable, elements like envelope (ADSR), filters etc.).

If you want to perform live music like One-Man-Band then buy an arranger.

If You want to play music in a band, buy synth (although arrangers, sequencers, and all others are in fact synths).

If You want to create some music (in studio) buy workstation (sequencer).

There are arranger able to create new sounds (with sampler, or add-on cards) but they are quite expensive (2000 USD - up), and mostly used by professionals (people who earn money playing like OMB).