See also Reaper hot keys
How to convert drums to midi with great control by installing a script?
Reddit and the original author both say Samplomatic5000 is "hard to beat for versatility" as a sampler: ReaSamplomatic500 Basic Tutorial and its web page
Useful video on the install and use of MT PowerDrumKit (download "VST" under Mac) and its VST instrument that can be inserted via Track -> Insert Virtual Instrument.
Upon dragging in a single drum loop and extending it in the editor, Bill observes that editing a single note changes all future instances of the measure. This is why it can be important to choose how many measures if one is going to choose a groove. Otherwise, if you make a short drum loop and drag it longer, you'll get lots more crashes and no apparent means to edit them without splitting each measure and editing them individually.
linkFix: Clicking a note adds to selection, instead of just selecting the note
This can be fixed by opening Reaper Preferences (Cmd-, on macOs, so presumably Ctrl and comma on PC), choosing "Mouse Modifiers" under "Editing Behavior," then selecting "MIDI note" and "Left click" for context, and changing "Default action" to "Select note":