If you're going on express trains like the German ICE, a 20" folding bike can fit in the luggage rack in the middle of the passenger car. However, you'll need a low rear rack, like the Tern Cargo Rack. I had a Speed TR, which is like a Speed D8 with the addition of a SRAM dual drive hub, but it came with a tall rack that required me to remove the wheels to fit in the ICE train.
https://www.ternbicycles.com/gear/471/cargo-rack
Normally you don't need to cover the bike when taking the ICE train but the conductors are kind of angry when they see a dirty bike. You can pickup a Tern Carry On Cover 2.0 for around $40 (kind of similar to the Dahon version). It's the lightest cover I could find at around 500g, and has a shoulder strap. The bottom and top can be opened to stick out the wheels or the seat post if you want to.
https://www.ternbicycles.com/gear/472/carryon-cover-20
If you're on flat land, 8 speed may be ok. If you're sticking to a standard 1x drivetrain, I would recommend easily changing it to 1x10, 11-36T cassette with the Shimano Zee rear derailleur. The Zee is short enough for 36T to clear the ground down to a 16" wheel, but doesn't have enough cage length to fit a 2x front. If you have more time, I could recommend installing a 2x10 drivetrain with a front derailleur if you're interested in DIY tinkering, since most Dahon bikes don't come with a FD braze-on. Some of the Tern Verge models come with FD braze-on. If you decide to go near mountains, you'll definitely need more than an 8 speed 1x drivetrain. I recommend at least 20-80 gear inches, which is a 400% range (although 11-36T is 327%). If you're not so fit, you might want to go down to 16 gear inches. As you get below 15, it becomes too slow that it is more convenient to walk. I cruise on flat land at around 67-70 so I recommend at least having 75. I rarely go over 83 unless I'm going downhill but it lasts for such a short duration that I would rather extend the lower range. At the moment I've modified my Dahon Vigor to 16-80 gear inches with a 2x10 drivetrain, which is around 500% range.
A SRAM Dual Drive would solve any gear ranging problems, but you need a 135mm wide dropout, and you have to check if it will fit the hub axle. Dual Drive is discontinued but you can still find them on eBay or aliexpress. middle gear is 1:1 and you lose something like 5% efficiency on low or high gear. If you go with a Brompton, it's got a narrow rear dropout, so you're stuck with their rear drivetrain, although there are people on other threads who've successfully installed a double/triple front derailleur with a 6-8 rear internal gear hub. In comparison I think the Dahon or Tern is easier to modify than the Brompton for a FD. There are also 2-3 gear BBs, like the Schlumpf Drive (2x), ATS (licensed), EFNEO (3x), and some others. But they make the low gear 1:1, and you also lose 5% on the high gear. It would have been better if they made the high gear 1:1 since you'd spend more time on high.
20" is the maximum size to fit in a standard suitcase while staying below the 156cm total linear dimension oversized limit. You can buy cheap hard shell suitcases for $50 on ebay. For 20" you need at least 45-48cm wide. 16" and 18" are easier to pack. 20" bikes need the wheels removed to fit in the suitcase. You also could separate the parts into 2 suitcases if you are willing to take 2. You could remove the handlepost and fork too, which is easy to do, but takes time to reassemble and adjust to the correct angle again. Some 16" bikes don't need disassembly, but if they do, it's just simply the wheels.
The main differences between the different brands of bikes are the folding size, how they fold, the stability of the hinge, and whether they can use standard components, and where you can attach racks and bags. The Dahon and Tern bikes would be easier to modify than a Brompton since you can use more standard components on them. Dahon and Tern are also much cheaper. 20" also come with more standard dropouts than 18" or 16" (for example, having RD hangars). However, if you are 90kg and want to carry a lot more gear, perhaps 25kg, you might be reaching the limit of most folding bikes. Some people may say it's still good though. I'm only 65kg. The bikes where the rear folds under, like the Bike Friday, Brompton, Birdy, IVE, might have less flexing at heavy loads compared to center hinges like Dahon and Tern. Just be careful that the IVE has narrow dropouts that is supposed to lower the load limit, but some retailers say they've changed the rims to make it stronger.
The Speed is steel with Dahon's "Vise grip" hinge. The one I had failed after a few years, but other people say their's are still perfect. Dahon also has aluminum bikes with Vise grip which are a little different than the hinges on the steel version. The Vigor that I have now uses their V-clamp which is the same hinge on the Mu. Tern's hings are similar to Dahon's Vise grip, and they make 2-3 different versions of it. Tern folds with the front wheel turned 180 degrees while Dahon does not. Tern has the front V-brakes behind the fork while Dahon has it in front, which makes Tern front racks easier to install and remove. Dahon front racks are difficult to find and require removing the font brake cable to install the rack. Tern front racks can't be installed on Dahon bikes because of the position of the brakes. Tern handlepost uses 2 bolts to attach to the fork stem while Dahon uses only 1 bolt. This makes Tern handleposts fold taller by maybe 2-3cm, perhaps not much, but may be important for fitting in a suitcase. If you're tall and need to have a more forward hand position, you can use the Tern Andros 2 stem which will add an extra 3cm forward compared to the standard stem that comes with Dahon bikes. you need to have a T-handlepost to use the Andros.
If you want better folding pedals, you should get MKS EZY. The EZY Superior is a newer version but has a wider diameter quick release so I find my shoes rubbing on them when I try to place my feet as close as possible to the crank arms. So I switched to the MKS EZY Superior Urban Platform, which keeps my shoes 1-2mm above the quick release. I had to shave off the sharp tabs on top of the pedals to allow my feet to get close to the crank arms. I had the Esprit but those with the open top with the Superior will make your shoes rub on the quick releases.