Check out the Tektronix TBS1000B series digital oscilloscopes.
- 50 MHz to 200 MHz
- 1 GS/s to 2 GS/s
- Record length 2.5k point
- 2 channel
For the budget constrained I’ve noted the Rigol DS1102E oscilloscope.
Book recommendation on the subject of TENS. It is available as a Kindle eBook.