Condensation is definitely a concern in colder weather. If you create a thermal break that isn't properly sealed against air leakage, warm air from the house will seep behind the walls, bringing lots and lots of moisture with it. Meanwhile the bricks, which are now insulated, are going to be very very cold, and when warm air gets cold it lets go of its moisture - and thus we get condensation.
That condensation would then get trapped inside the walls - and long before you have a problem with brick deterioration you're going to have mold issues that lead to health issues.
The best thing to do to prevent this is to insulate with a substance that also serves as a vapor barrier, which will prevent any air (and thus any moisture) from leaking into the closed cavity. The best product for this IMO is spray foam insulation, because it fills in all the nooks and crannies and IS the vapor barrier.
Alternatively, you could use fiberglass and cover it over with plastic to form a vapor barrier, and this can work IF you properly seal it all the way around the edges, seams and any holes that occur.
Either approach, however, is going to require you to sacrifice living space, as for a proper thermal break you're going to probably want at least 4" of insulation.
Now - with regard to insulating your spare rooms. Don't insulate your interior walls. Put proper insulation in the external walls of those rooms, so that they don't lose heat to the outside. Yes you will be heating them, but if they're properly insulated they're not going to cost much to heat and there are numerous other concerns with turning an inside room into a "cold" room - it's a finished space, after all, and you don't want to risk damaging that by letting it be freezing cold in there.
If you are foaming the attic ceiling, you're converting the attic to conditioned space. If you do this, you must cover up the ridge, eave and gable vents (or any other vent, for that matter)
Here is an information link on the subject:
http://inspectapedia.com/interiors/atticcond8.htm
Best Answer
Skip the expanding foam and buy pre-made foam panels from your local HI store. Layer them up for more insulation and cut them to size with a knife or saw (I just use a drywall saw). You can enclose them in a wood box on top if you'd like, but they're rigid enough that they'll stay where you leave them without building an enclosure (unless you turn the fan on).
Just saw your edit about the motor. You can build up the foam panels into a tent. But if you can get a cover designed for a whole house fan, that may be your best option.