Generally a car that is interesting due to engineering\ provenance, or rare, or both. Some of the limited run cars from Aston Martin, Mercedes, Nobel, etc. Many of the small manufacturers are likely to produce cars that are kept an maintained.
As for the question on parts: if there is demand, someone will start to make pattern parts. Older cars nowadays have to have parts made for them, or located from stock of old spares in storage somewhere. The increasing complexity may make this harder, as a wider variety of parts need used, and they have to be made to finer tolerances, but there is no reason our engineering base cannot keep up.