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.
Roxul is great stuff. There are 3.5" and 5.5" depth batts. With a combination of those, you should easily be able to entirely fill the stud cavities (leaving no voids inside is important). You can use a bread knife to really shape the batts to perfectly fit around obstructions and make slits for wires and things like that to get a very neat installation. The material is super DIY-friendly, and much easier to work with than fiberglass batts.
As for your windows, if you like them and they're attractive and in good condition, there's no reason to replace them; just get storm windows with your desired glass characteristics (laminated, Low-e, etc).
Best Answer
It IS possible to insulate with a "wrap" on the outside, but then you need to extend all the window frames: a huge undertaking, and really hard to seal properly.
Your practical options are blown in insulation from the outside or from the inside. Then you can choose from fiberglass, cellulose (most common) or maybe maybe foam.
From the outside is less disruptive, but introduces more chances for leaks. From the inside works OK. Two holes are drilled per wall cavity. You'll want to use a contractor who uses a thermal gun, to ensure all the cavities are filled.
As mentioned here: Alternatives to conventional weep screed, for stucco to the ground homes? the stucco and interior plaster of that era is just reaching maximum strength, and it's rock rock rock rock hard. Drilling from the outside is a serious chore.
You also have a SEVERE danager. Existing leaks in your stucco are no big deal, they dry out. With blown in cellulose in particular you now have wet compost in your walls. Paying a professional energy consultant is needed to give a better answer and to really evaluate your house, habits and opportunities for energy savings and comfort upgrades.