Can't comment on the enlarger, but the Nikkor 75mm should cover 6X6 fine. For larger 120 negs, especially 6X9, you may wish to have a longer lens like the 100mm you saw recommended elsewhere. The particular enlarger does not affect the necessary lens length, but it will effect whether you need to have a recessed board. Generally speaking, however, usually longer lenses might need an extended board and shorter lenses a recessed board. Some enlargers have bellows that allow you to use a large range of focal lengths with flat boards, sounds like yours does not have that flexibility.