1 - with the 850Kv motors and 10 inch props the motors will be able to produce about 2.8Kg thrust. Given that you want to hover at about 50% thrust the total weight of your arducopter should not weight much more than 1.6Kg. I cant recall of hand what the weight of the quad frame is , but in general if you just need to add another 100g then the basic setup will be fine :-). To learn more check out our guide on what motor you need for your drone.
Here are the motor test data table for the arducopter 850Kv motors should you be interested:
2 - This is slightly more tricky, but the best way would be to simply talk between the two via MAVlink protocol which has python bindings if you want to work in python. There is actualy a great guide on how to connect your ardupilot/arducopter to a raspberry pi here
3 - My suggestion would be to use a high power wifi router on the raspberry pi. Most of the off the shelf versions only work to about 100m, but there are some other ones available that should go further but it depends on how much you want to spend, and how much weight you have. What sort of range would you need?