I've been try to tab out some piano, (standard notation and tab) on guitar pro, but I can't get low enough to the notes I want to use. I've tried changing my track to a bass clef and everything! Is there just a programme dedicated to tabbing out piano (that can reach the low notes of a proper piano?
Why not seperate the left and right hands into different tracks, and adjust the tuning of the left hand so its far lower. That way you can surely reach the wide range you are after.

Otherwise, Sibelius and Finale are the obvious choices, but they arent cheap.