1. A standard 5' tub will fit. You will most likely need to replumb the drain.
2. Acrylic won't rust, but t doesn't retain heat like a cast iron tub. Steel tubs don't retan heat well either.
3. How close to the existing tub is the toilet. The new tub cannot come closer than 15" from the apron to the center of the toilet. Generally speaking a "deep" tub will als be wider, unless you are only talking about 1 or 2 inches
If they plan on taking baths then cast iron is the best. It lasts a lifetime and holds heat better than other materials. Can be laborsome setting cast iron though, a 5' tub weighs about 350 lbs.