Thank you all for your helpful responses. Like Peggy mentions, PLA might be a good option, since as a polymer engineer, I know this as a rather "soft" plastic in contrast with other "difficult" to handle options. As I have not yet the printer, what I'm gonna do is to get some PLA filaments, make something manually and then prepare the corresponding gypsum-plaster mold. A good mold should result in a flawless piece in the end, that's why I will then pour the mold with silver to make sure about the quality of the final product.
Form 1 is also a very good option as far as I can see but I will check Ultimake first. Btw, thank you for all nice recommendations. I will post the results as soon as I do the casting. Cheers- Ali