Hoosier,
The biggest negative of these cars is "do you fit"? If you are tall (I'm just over 6' 6"), and/or have a pot belly (I don't), you won't be able to get in and out of any year of these cars easily. The steering wheel, even though it telescopes, will not allow it because it is so large in diameter. The problem can be corrected with a smaller steering wheel that several aftermarket sources sell, that is a smaller diametered copy of the originals and looks stock.
To get into the car, back in and sit down before you swing your feet in. Otherwise you can't get under the steering wheel, and you'll smash your knee on the bottom of the wrap around windshield that juts into the door opening.
The cars tend to run hot and overheat in slow traffic and on very hot days. The problem is rust buildup in the engine block of poorly maintained units over the years. Add to that an engine bay that is not well ventilated, and unleaded fuels that can boil on hot days, creating a vapour lock, and you can find yourself waiting on the side of the road for things to cool down. There are fixes for these issues, electric parade fans, engine block flushing, high flow water pump, all easily found on line.
The 55 is a 6 volt system, which many upgrade to a 12 volt system to get easier engine starting. Many 55's have had the upgrade, those that haven't, the upgrade is fairly cheap to do. 56 and 57's are all 12 volt. My wife likes the power seat that came standard on the 55.
Cars came with either a hard top, or soft top, or with both. Get a soft top, because its far more convenient than the hard top, which is hard to store, and a two person job to mount/unmount. Lowering/raising the soft top is a two person job as well.
Buy the BEST car that you can afford ... and have its body work checked out by someone who knows what to look for ... plastic body fill and rust repairs. Because these cars are so sought after and iconic, they were almost never put in a wrecking yard after a heavy collision. I personally know of three small birds (I worked in a small bird restoration shop until the owner died several years ago) that looked very nice when purchased by their owners used. One 56 had a complete rear tail section grafted onto its back end about a foot in from the tail lights. Another 57 had a new frame and either an entire front or entire rear grafted together straight across the car midway through the door openings. The third one, a 56, was burned and consumed in a garage fire. The new owner of the hulk rebuilt it using parts from many Fords of that same era, even using the same warped body panels. There are a lot of horror stories hidden under the paint. Check it carefully.
The 55 and 56 are almost identical. The 56 added small air intakes to the passenger foot area, and had a stretched and beefed up frame for the continental kit hung out the back end. This got the spare tire out of the 55's trunk, which owners complained about taking up too much space. Many 56's had rear spring sag due to the added weight. 57's were stretched in the rear end to put the spare tire back inside the trunk but standing up out of the way.
57's are the most sought after, due to their styling.
I can't stress enough to have the car you're considering checked out by an expert in small birds. And it is always cheaper to buy one that is restored, than to buy one that needs restoration. Restoration costs are very expensive at today's shop rates. Some body parts are as well. Rear quarter panels were selling for $7500 each a few years back.
Good luck in your search!