The captor area will depend on the size of the heatpump needed to heat the house.
You are right about the 1m dept, it can be deeper, the more soil on top, the more heat retained. 1m apart I don't think is too far off, more like 2 ft.
A brine system will require more piping than a Direct expansion (DX) system.
It takes up a lot of room alright, off the top off my head I think ours is about 30ft wide and 90ft long.
18k for the drill hole sound way off, only way to find out is price it.
Air sourced, I personally am not a fan after seening how muh they cost people to run, but this is also down to poor installation. ASHP are the least efficent, if u can afford a bore hole or vertical heat pump I would go for it, if not then geo, Air Sourced would the last choice.