This might not be nearly as pretty, but if you're just looking for a low-tech keyboard that ultimately can link to the internet, you can get one of these (Amazon link) for $20 used. It's called an Alphasmart 3000. Like the picture, it's a low-tech keyboard that mainly does just word processing. It does allow copy and paste, but not a lot of it. From the Amazon reviews, it looks like people are still buying them. I don't know if they're still being manufactured.
I'm not sure what a null modem cable is, but the Alphasmart uses USB. It has to link to a computer. The Alphasmart is a straight keyboard. It doesn't have any computer capabilities except word processing. A pre-laptop portable computer has me curious. How long between the time that the Armstrad NC 100 came out and the laptop was available?
A null modem was a kind of serial cable you could use to transfer data between two computers -- waaaay before USB! The NC 100 came out in 1992, so although it is a full computer, it doesn't really do anything we'd expect in a computer today. I think 1992 puts it at about the same time as the modern laptop, but it would have been way lighter than any laptops available at the time. So yeah, if I ever want to get my writing off the bloody machine, the Alphasmart is probably a better bet these days.