Re depth of field: When producing "same size (on film) head shots" depth of field wil be the same, no matter what lens.
Since medium formats are bigger, a full frame headshot in MF will have higher magnification than the same in 35 mm format. Hence the shallower depth of field in MF. But fill the frame with the same head, using different MF lenses, and you will not be able to notice any difference in depth of field.
The rough (!) 35 mm format equivalent of the 150 mm lens is 94 mm, the same of the 180 mm is 112 mm.
Looking at MTF curves is informative, and ican be quite usefull, but no matter what diferences these curves may show, fact is that both 150 mm and 180 mm lenses are more than good enough. No worries there. Not a deciding matter, in my view.
The 110 mm, by the way, needs to be stopped down to f/5.6 to even begin to come close to the 100 mm. Buy it for the shallow depth of field, and its very own character (which decidedly is a bit soft), but not to compare it to the 100 mm.
The 120 mm, on the other hand, is a very good choice too, if you like the a-bit-closer perspective. I do, but not for tight head shots. A 250 mm lens is my choice for that type of portraits.
While the 110 mm wide open is a bit soft at all distances, the 120 mm, is not (!) soft at all, not even at infinity.
The 135 mm is indistiguishable from the 120 mm, save for the small difference in focal length (not much) and the need (!!!) to use the 135 mm on a bellows or variable extension tube. If you're thinking about getting either 120 mm or 135 mm Makro-Planars, the 120 mm is the one to go for.
CB lenses, by the way, can be used on all post-1957 focal plane bodies except the 202 FA. So "not for most focal plane bodies" really should say "for most focal plane bodies".