Thanks for your comment! I'd definitely recommend getting hold of these machines, but there are things you have to look out for because of their age. The issues I can think of right now are: (i) check for evidence that it works; in online sales you want pictures of the machine switched on, with a screen display lacking blotches or missing vertical pixel lines, (ii) check that the hinges and surrounding area aren't cracked or broken.
If you find a machine that looks fine there might still be minor problems with it that can be worked around. The machines have a metal tongue that sits between the two AA batteries, and in a few of my machines that's broken or missing, but you can still power the machine up by inserting a piece of foil between the batteries to make the contact. The batteries are awkward to put in with this problem but you get the knack of it soon enough.
Since these machines are battery powered, all you really need are the computer itself. But to get 3rd party software onto them you'll also need the 3-link, a serial cable with a dongle in the middle containing a ROM with the comms software (a Series 3c or 3mx don't need the dongle as they have the software in ROM, but their cable still has a bespoke connector at the Psion end). And to connect to a modern machine you'll need a USB-to-serial adapter.
You'll probably want memory cards for storage too, proprietary cards called SSDs (solid state disks) on the S3 family. There are two types: Flash SSDs, where space is not freed up by deleting files and are therefore more suitable for archiving or storing the software itself, and RAM SSDs that work like modern storage and are therefore good for files that get saved often. I especially recommend a Flash SSD for installing software to so that it's not taking up main memory, especially on machines with 128k or 256k of memory.
I hope this helps; if you need more information let me know!