Should I rescue this? '66 Dart 4-door

Depends on how restorable it is. If it's fundamentally sound, $600 is a fine deal for what looks like a straight, high-spec (factory air!), mostly complete car. If it's got terminal undercar rust, then $600 is a fine deal for a whole lot of choice parts (complete factory A/C system, desirable cable-type '64-'66 A-body accelerator pedal setup with kickdown linkage, front and rear bumper guards, very restorable steering wheel with full horn ring, etc).

"Super 225" was the decal applied to '63-'68 Plymouth 225 air cleaners; the Dodges of that era with that engine got "Charger 225" decals. It doesn't signify any special engine or power package, and it's a 1bbl setup. The '77-'83ish 2bbl package had a "Super Six" air cleaner decal.