Best thing BMW can do is price it around 55k so buyers have plenty of financial leeway for additional options without breaking the bank. Options could easily add another 5k to the final price depending on what you want in your Z5.
They should not price it anywhere near the 718. While BMW is a name associated with luxury, Porche is regarded as a brand that is above BMW. If the Z5 is priced similar to the 718, people may as well go for the more highly regarded brand.