The 50cm is too big if the seat is as low as it can go and it's too high for you. Be aware that some bike shops will try to sell what is on the floor even if it doesn't fit properly.

People have different proportions, so height is not the only thing to look at. Inseam is also important, as arm length and flexibility.

I second that you should ride the symmetry and get it checked out before buying it. If it doesn't fit, it doesn't matter how cheap it is.