Is it permissible for me to ask a girl if she would accept me if I proposed to her, knowing that she is two years older than me and I have sought God's guidance through Istikhara?

There is no harm in addressing your colleague and informing her of your desire to marry, provided that you observe the Sharia guidelines, such as not being in seclusion (khalwa), lowering your gaze, and limiting yourselves to permissible speech. It is permissible for a man to propose to a woman directly, even if she has a guardian.