The last time I had an ECG taken (some years ago now), they used a toy* that was not much bigger than a PDA (of that era).
It may have been this
... and the A4 printouts it generated were entirely adequate, I should add!
You can check the manual here
Numed used to supply them (not sure about now) - but I note that they also offer a PC-based system
(which you may also wish to consider; you can rent them as well).
* It reminded me of the first GameBoy!