I can't answer all of your questions without drawings. (I suggest hiring an architect at least so you can see what the changes look like.)
It sounds like you have a lot of land (or at least, enough..) and a detached garage.. From this, I gather that you live in an attractive neighborhood. So here's my only comment - don't ruin it with a giant garage in front of the house. Is there any way you can enter from the side or back of the house? ..or consider a carport, if you just want to keep snow & rain off the cars.
.. see an architect. With changes that big, it really will be worth it.