B - There are plenty still flying. There used to be a lot more actually, as they were sold off the public quite cheaply after the war. Many many have been lost to accidents over the years - The P-51 is NOT a forgiving airplane.
C - The biggest challenge - Really the ONLY major challenge, airframe/structural work is basically a solved problem - are the engines. The Rolls Royce Merlin engine, used in both the Spitfire and the P-51, was last manufactured in 1950. While there were many built (and thus many to scavenge from), that inventory is only shrinking as engines are replaced or overhauled. Basically the world's entire stock of Merlin parts lives in a group of semi trailers in the California desert (http://www.51-factory.com/merlin_overhaul.html) - when they're gone, they're gone.