First is The Last Man On Earth with Vincent Price. It's an American production, filmed in Italy and based on the novel "I Am Legend" (later made into the much dumber "The Omega Man" with Charlton Heston). The Price version explores the unthinkable situation of a single man fighting off an entire planet of humans changed to vampires. The question the movie poses is "If everyone is a monster but you, then doesn't that make you the monster?"
Also it features Vincent Price doing something other than the campy "sinister man" character that comprised most of his career. It's a truly excellent film that deserves a better fate than dollar-store obscurity.
The next is Dementia 13. It's Francis Ford Coppola's directorial debut, produced by Roger Corman and quickly filmed in Ireland during some downtime while filming yet-another Edgar Allen Poe adaptation. The story's not as tight as "Last Man On Earth", but the film makes up in story with influence. I don't think there's a frame of this film that wasn't shamelessly copied by horror filmmakers for the next 30 years. The people who directed every slasher film made since 1963 owe Coppola a hundred bucks each for coming up with all of their techniques.
And if you're at that dollar store and you find the original "The Little Shop of Horrors" and "Bucket of Blood", grab those too. Despite being only an hour long each and very cheap, they're tons of fun and are just what you need to round out your Halloween film festival.