I wrote things up in the following threads, mainly recommending the Schneider Kreuznach 120 Marco Symmar (see the second link for the right magnification level) or the Rodenstock Apo-Rodagon D 2X 75mm/4.5, which is the consumer version of the lens inside the Flextight scanners. Both lenses cover digital medium format easily, so can’t go wrong on either of them.