Most critical consideration is the "rough-in" dimension for your existing toilets. This is the distance from the center of the toilet's outlet to the rear wall -- usually the same as the distance from the 2 bolts holding the toilet down to the wall. The most common rough-in is 12", but 10" and 14" toilets are available.
Other than that, it's largely a personal choice. Read Terry's report on low-flow toilets (http://www.terrylove.com/crtoilet.htm) for a good overview. Many of the forum participants prefer Toto toilets, although there are some rebels. I bought a Toto Drake (ADA, elongated) largely because of what I read on this forum, and am very pleased.
As for tools, etc., if hubby has removed and re-set toilets, he's probably got everything he needs. I would plan on replacing the riser pipes with flexible lines, if the old ones are chrome or copper, and you'll need new wax rings, but otherwise it's pretty straightforward. If all the stop valves work OK and turn freely, you can continue to use them, but if they need to be replaced, now is the time.