How safe is it?
It would really depend on where you will buy the "bitcoin" from but you need to remember that you do not actually own the bitcoin you will buy but instead just a representative or a reflection of the real bitcoin. The "bitcoin" you will buy is regulated by an authority and therefore you do not actually have full access nor control over it which is to me not safe since I would want to be the one to hold my actual bitcoin.
How dose the price differ from BTC its self?
What commision do they charge?
Each market has some differences due to real time tracking errors. It is possible that the price in one market appears a few dollars short or vice versa. But with stock exchanges there is usually the broker fee you have to consider. I am pretty sure it differs per each one so you are going to need to choose the best one.
May I ask why buy in stock exchange though??? Why not just buy in a crypto exchange if you really want a centralized authority holding your coin