Before you buy a DSO138 or DSO150 from eBay, Amazon, wish or Ali Express, it is important to understand what you are getting. The specifications are very loosely written and what looks good on paper is useless in practical use. Here I test the DSO138 specified at 200 kHz, but in reality it is worthless above 20 kHz.
Buying the right oscilloscope is knowing your work area limits and where it might be in the future. I recommend that you read: 12 things to consider when choosing an oscilloscope: https://www.mouser.com/pdfdocs/Tektronix12_things_to_consider1.pdf
For the Nyquist applet to play with: http://184.108.40.206/applets/AppletNyquist/Appl_Nyquist2.html