Don't buy any books before school starts. During hospital day, some of the club's that you join will give you books as part of joining the club. These include the Netter Atlas and Goljan's for Pathology. I would say that the only books you really need first year are Netter's and First Aid--not to study for boards, but to look into the book and see what correlates between the board book and the class material. Most of the time, each exam is over 10+ lectures and over 500-600 slides, so it can be a challenge to figure out what material is "high yield". With First Aid you get to see which topics are important for whatever class that your are studying for. Plus, there is so much information that you seldom have time to really read any supplemental or "required reading" for each class, so it is really not ideal to buy a bunch of books.
As far a rough estimate for cost...I would say around $1000.
~ 700 for equipment
~ lab gloves, Dr. Khan's dissector, and dissection equipment ~ 100
~ OMM shirts, shorts pants ~ 70
~ Netter & First Aid ~100