The TRS-80 Model 100 portable officially allowed for RAM expansion to 32k and an external acoustic coupler for the model. What other upgrades and peripherals exist for the Model 100? I had one of these second hand, and they have great battery life and true portability (about 3 lb) unlike most of the 80s portables which weighed 16 or even 28 lb!
As you mentioned, you could upgrade TRS-80 Model 100's RAM to 32k. You can also get a REX chip, which uses the ROM expansion slot to provide additional flash memory.
The TRS-80 Model 100 had several peripherals available (in addition to the acoustic coupler) including a cassette recorder, an external drive, a barcode reader, printers, and even a "Disk/Video Interface" that allowed you to connect up to two 184k disk drives and a TV or video monitor...
You could even use a "null modem" to connect the 100 to other TRS-80 computers to transfer files.
Here's a link to the user manual. The peripheral connection guidelines are in Appendix A.