What's the max level in the steam version of the game (patch 2.5 I believe)? I know that the answer should be easily googleable, but I only found the number "122", which is incorrect at the moment (I am at 129 or 130 right now).
It appears to be 134.
It's 134, just checked. Max level will always be the final upgrade of the latest character.
The max level depends on the number of Awesomenauts available. Currently, the max level is 138, with the patch 2.8, then 144 with the patch 2.9. If a new Awesomenauts is released in a future patch/version, the level will increase by 7 or 8, to unlock new upgrades for this Awesomenauts (plus some empty levels, sometimes).
By accessing to the max level, you can unlock a level of prestige.
With release of Awesomenauts 3.5, the level requirement was 192.