Well, if you can keep the Cases, that's what I would do just because they are super-cool. You might search online for seats, because there are original and aftermarket seats out there for an enormous array of oddball toilets. There are manufacturers that just make seats for specialized toilets, like my funky lowboy, in all sorts of original colors, and the contrast of a white seat or a dark blue, etc., might also look cool.
The Case hangs on a standard carrier, with one part added to it and two bolts removed, according to posts from our resident expert on them. As I understand it, you open the wall, remove the hanger piece from the carrier, and install a bolt in each of the two holes in the carrier that arent' being used, and voila. Ready for your boring modern toilet.
The Glenwall is pressure-assist and works well. It uses the Sloan Flushmate pressure device, which has proven generally to be reliable (although some earlier versions occasionally exploded). You do get the DOOOOSH! sound that any pressure-assist makes. Some people love that, some hate it.
The Maxwell is a gravity flush, similar to most toilets these days. Terry has installed a bunch of both at one vacation condominium complex in mountainous Rural Washington, and he says that the Maxwell is quiet and flushes well. I can't imagine that it's gonna win any awards for flushing, but it is apparently satisfactory. A lot of his clients have been installing it. Cheaper and quieter than the Glenwall, but the Glenwall has a very effective flush. (As with many pressure bowls, my experience is that it will sometimes pulverize the toilet paper and leave little teensy bits of pulverized paper in the remaining water. Not always, but sometimes.)
FWIW, you probably know that you can't put a gravity tank on a pressure bowl, so if it's too noisy, you need to replace the whole Glenwall, tank and bowl, and vice-versa. Every once in a while, someone comes on here to say they removed the flushmate from the tank and installed nice Korky gravity parts, and now when they flush, the water just stays in the tank. Well, yeah. If you put a flushmate in a gravity bowl, it will spray all over, and if you put a gravity system in a pressure bowl, it will just stay in the tank. Two different bowl designs for two different flush systems.