7.5 volts DC should be enough. If it needs 12V something is wrong. Are you sure it's not AC but Direct Current, with the polarity correct?
The MAX232 chip should not get hot. The only chip on the board that should get warm (60c is reasonable depending on the supply voltage) is the 7805. But nothing should get hot.
C could be used but you need a compiler. You can't use all of the features due to limited space, but a proper compiler should have a help file with that information. I'm not sure about C++.