Is it permissible to advise a girl online regarding her hijab and dress before proposing to her, so that agreement can be reached on শরعي (Shari'i) matters prior to the official engagement?

There is no harm in advising a girl and proposing marriage to her, while observing the regulations for speaking with a non-mahram (definition) woman. It is recommended to choose a religious woman, as the Prophet, peace and blessings be upon him, said: "then win the religious woman..." And because the best thing a believer can benefit from, after the fear of Allah, is a righteous wife, as stated in the hadith (definition): "The world is a provision, and the best of its provision is a righteous woman."