Looking for advice on wireless install

Yes. You can do it with two wireless access points. Locate one access point at where the hallways hit the connecting corridor. Hang them from the ceiling. Cisco has ceiling mounting kits that hang on the frames used for acoustic tile. Use either omnidirectional 5dBi antennas, or two 8dBi directional antennas with a power splitter. Aim each 8dBi antenna down the longer hallways. There should be enough side lobes to deal with the connecting corridor.

The problem is that this will only covers the hallways, and not the connecting rooms. If you plan to have access inside the rooms away from the hallways, you'll probably need one access point per room, if the walls are block, concrete, and steel rebar. Illuminating just the hallways isn't going to work very well. At best, you might get some coverage if you leave the doors open.

There are only 3 non-overlaping channels (1, 6, and 11). I've had some luck using 4 channels (1, 5, 8, 11). If you need to use more than 4 access points, you will have some channel overlap or duplication. Try to put the access points with duplicated channels as far away from each other as possible.

The same or a bit more than what the clients are using. You want a symmetrical system, where clients and access points both have roughly the same range. That works out to about 50-100mw for the access point.

As far as possible? I can't answer that without knowing more about the coverage area, and the exact building construction. My experience with concrete block buildings is fairly dismal. I usually illuminate those from the outside, entering through the typical large windows. One access point outside, with a carefully designed antenna system that only "illuminates" the side the building (not the entire neighborhood), works quite well if the windows are penetrateable. (Watch out for aluminized mylar tinting). However, when I try to illuminate the rooms, from a right angle, through a hallway, it never quite works right.

A building that old may have been constructed with mettle lath covered with plaster. That's going to be next to impossible to get adequate coverage. into the offices.