It's hard to advise you from the information you provide. When you mention "designs", that covers a lot of ground. The software that is good for producing, say, a functional prosthetic hand is not the same as the program you'd use for a lithophane or a Yoda head. But all these things can be printed. It's understandable that you want to make money, but how much do you want to spend? Depending on their capabilities, 3D design programs vary widely in cost. Instead of starting with what you want, try to think of what a customer might want, that's not already available. Like the cement company says: "Find a need and fill it".

Andrew Werby
www.computersculpture.com