Personally I think the "jump" from 60mm to 120mm is too great: it is 2x. IMHO, a "better" multiplier would be between 1.41 to 1.75x. This could give you a toolkit comprising of three evenly spaced lenses such as 60mm --> 100mm --> 180mm
Note: there are many many good combinations that would provide you a small toolkit consisting of three evenly spaced lenses. For example, with my Mamiya 7, I have a kit consisting of 50mm, 80mm and 150mm which covers most of what I'm after. Similarly, my Technikardan kit consists of 47mm, 75mm, 120m, 360mm, 500mm -- five relatively evenly spaced lenses with an added bonus: all use the same 67mm filters.
Anyway, there is no one perfect kit. Assuming you agree with this approach you might want to consider getting the 180mm now with an eye towards obtaining a 100mm lens later on (you should be able to sell your 120mm for a good price which should cover the cost of the 100mm).
Cheers