It's really tough to predict what kind of range you'll get. Some people have reported fairly short ranges on the order of 50-75 feet, while others have had much better success, with much greater distances.
Placement of the M1XRFTW matters. Generally, placing it in a central location on the first or second floor is better than at one end of the house. I like putting the receiver in a closet above the closet door as a way to keep it out of sight and out of the way.
I would try one receiver to start with and add a second to cover the house if you can't get full coverage with just one. You might even be able to reach the shop with the receiver in the house. It's all going to depend on local conditions.
Is there a reason you're going with the M1XRFTW rather than the GE or Honeywell wireless? I chose the M1XRFTW because I liked the idea of the two-way operation and the use of spread spectrum technology. But I've been disappointed by the lack of many sensors that are available for the other systems and the higher cost of the Elk sensors.