Welcome to the Team Mel!
As pointed out Chevy stuck the Impala name on a lot of cars made from 1958-'85, then reintroduced the name plate as the Impala SS in 1994-'96 being the last of the rear wheel drive V8 powered cars. The new V6 wrong wheel drive used the Impala name in vain from 2000-present.
If you buy a 1977-'89 model year Impala/Caprice you can have the Impala or Caprice name after 1986 in a "mid sized" car that was the same size and used a lot of the same parts as the earlier 1970-'75 Chevlle did. This way you can build a car that out performs the earlier Chevelle but in a "sleeper" body. Of course the only other use for these years Impala are in movies being crushed by car vs train wrecks or in car chases where fifty or so Impala police cars are wrecked chasing the amiable and lovable goofs who aren't really all that bad, just wanted by the law for some felony or other. These model years are usually available for the price of the scrap metal that they are often sold for. After 1982 there were no two door Impala's made, so if you want one you need to look at 1978-'82 model years for this series.
The single most popular year Impala is the 1964 followed closely by the 1966 model year. Of course every one wants a two door even though there were more four doors made than two doors because when they where new no one but a few die hard muscle car fanatics wanted one.
The four door hard top or four door sedan was much more particle for a family car. Of course in 1959 and 1960 the El Camino car/pickup truck was actually an Impala two door wagon minus the top. So they are fairly unique and rare, but then I am also partial to the early wagons with the rear glass in solid steel making it into a sedan delivery looking for a surf board and a Beach Boys song. Finally the 1958 is a unique series of only one year sharing body parts with no other year. Which was unusual, but a direct result of an internal GM mandate to share as many car parts across the marquis as possible with only the motors being different amongst the makes.