Buy these while you can, they're not getting any newer or any more common.
1. Volkswagen Beetle; pre-1967
Little needs to be said about the Beetle except that pre-1967 examples are getting scarce. Parts are readily available from a number of sources so restoring one is easy (albeit not cheap). Look for an unmolested example with minimal rust and hell, drive it as-is, even every day. Post-1967 models are interesting too but not (yet) worth what the old ones are.
2. Alfa Romeo GTV/Giulia; 1964 - 1974
These cars have skyrocketed in value recently but you can still find examples that need minor work for less than $10,000. There aren't enough to go around so if you find one, go for it. Most issues can be fixed, even if the engine has to be swapped or something similar. While you don’t have the factory’s support for parts, you can find most anything through aftermarket vendors that offer quality reproductions. The DOHC engines are reliable if well-maintained, though the SPICA injection in later GTVs can be a pain to tune correctly. They’re fun to drive (like any Alfa should be) but it goes without saying that they rust, look for it in the rockers and in the floors.
3. Citroën ID/DS; 1955-1975
Citroën’s legendary sedan, they were imported in America for a short time and if you look you can usually find one for sale. The twenty year production run means you can find one to suit almost any taste: automatic or manual, carbureted or fuel injected, etc. The hydraulic system is intimidating but not impossible to figure out; it’s nothing compared to modern car electronics. Parts are readily available but they’d probably have to come from France. Once restored these are arguably the smoothest-riding sedans on the road today. Bonus points if you can find a Safari wagon.
4. Mercedes-Benz w108; 1965-1972.
The w108 S-Class line was made up of convertibles, coupes and sedans. The sedans can be picked up for cheap and are simple, reliable and you can get almost any part new through the Mercedes-Benz Classic Center. The 280SE 4.5 is a particularly popular option due to the 4.5 V8 under the hood. Provided you can fund its gas habit this can be a reliable daily driver that is slowly appreciating in value.
5. Lancia Beta; 1972-1984.
Front wheel drive Fiat-based cars, they're reputed for being unreliable and for rusting quickly. They live up to the reputation for rusting but the unreliability is unfair and a typical bias against Italian cars. These cars use DOHC Fiat engines which are robust if maintained correctly. They come in a variety of flavors including coupe, Zagato, HPE, Monte Carlo and the rare sedan; all are equally fun to drive. The Fiat drivetrain means that engine parts aren't too hard to find but Lancia-specific parts can be tricky. Find a rust free, complete one and work on the engine rather than one with a good engine and a bad body. These unappreciated cars are the next Lancias to go up in value.