[Griefbringer]

I did a little test, and managed to purchase units with starting experience of up to 140.

I simply turned off the historical training preference, set the quality to 120 and purchased US ranger scouts (formation experience modifier +13). Thus the units purchased had their starting experience in the 120-140 range, with the highest scores being exactly 140.

That said, I have no idea what are the practical possibilities of getting the experience beyond 120 in an actual campaign play. The highest experience rating I have managed to reach so far has been 116 in a French long campaign (Char B1 with 82 kills).