Questions of Setting

So as usual I am juggling way too many story ideas than is healthy or recommended by most qualified health professionals. What can I say, I'm an addict. Along with recently revamping my efforts of my Kouros Series, I have also just had inspiration for a stand alone novel. And while its characters and their general plot line is clear, I have no idea where to start the story. As in where in time and space. Because the story is the story of two lovers, who are cursed. Cursed to love each other for all eternity. What that means is that when they die, they are reborn. But only one of them remembers their previous life and the love they had. Which means it is up to this one to find his lover in the sea of humanity. But the catch is, after he finds her and gets her to fall in love with him again, and tells her about their previous lives together, he will always die soon afterward. And then once she dies and they are reborn, she will be the one that remembers, but he won't. And so it starts again. So I'm tempted to start this story 'in medias res' from a third person perspective and then switch back and forth from being in one of the lovers minds to being in the other with each subsequent rebirth. The other option is that I can start the story chronologically and keep it in first person narrator with complete or limited perspective and end it with the narrator of the story actually being one of the lovers, telling the other lover, wherever she may be, their story. Because humanity has grown too large and finding a single individual nearly impossible. So he wrote the book with their entire story in it in the hopes that maybe she will pick it up and read it and remember and come find him. In which case the entire book, how its packaged and presented and marketed will be as if this narrator and character is actually the author, is actually real. So those are my two options. The other thing is just how far back should their history go? 1700s? 1400s? B.C.s? Ancient Babylonia? Also, would you as a reader be content in not ever really finding out how they were cursed, just kinda going with it as a part of the established rules of the fictitious universe? Thanks for the feedback and ideas. I really appreciate it. :)