Quantity   Component name
1 × Raspberry Pi Zero You could use a Zero W if you like!
1 × Psion Series 3 (or Acorn Pocket Book) Later models (3a/3c/3mx) have a larger screen, but you'll need a different link cable for the 3c and 3mx.
1 × Psion 3Link cable Have a look on eBay; Acorn's equivalent version was called the A-Link.
1 × USB Hub (2 ports or more) Use one that delivers a decent current otherwise it won't be able to run the dongle and the USB-serial converter at the same time
1 × USB-to-serial converter They go for a few pounds on eBay. Take care with them as they tend to be fragile
1 × Power bank Make sure it's good quality and outputs a decent current (1A upwards?) Also make sure the capacity is decent - mine is 6000mAh.
1 × Nice case (optional) If you carry a bundle of wires around with your Series 3, you will look unusual, and not in a good way.