All my answers assume you do not have a Hopper system.
1. If you buy a receiver make sure it has not been discontinued from being activated, many older ones no longer will be allowed by DISH. In addition make sure it has no balance due on it, or you will have to pay that before activating. Finally make sure it isn't stolen. That can call be accomplished by contacting a DIRT member here and they will check for you.
2. Getting an additional receiver from DISH does not necessarily mean you have to sign a new contract, if you don't want to buy one. Again check with a DIRT member here to let you know based on your account. As mentioned by JM42 one advantage to buying the receiver is you can have it turned off or on anytime. If you get one from DISH if you have it turned off it has to be returned. If you plan on the receiver being used just as it is in the house and not seasonally I would see about getting one from DISH because the cost to you for monthly service is the same either way.
3. No one here can tell you if you can run another line off your DISH till exactly what equipment you now have is known. In the end if you plan on using the garage receiver often, if you can't make it another outlet easily it may be worth having a new DISH installed or equipment added as needed with the outlet in the Garage.
Much depends on how far away we are talking the garage is from the DISH, where the cable would be run etc....