For me, it was perhaps simple-scan, a very simple and efficient GUI to scan documents. I used it with my Brother printer / scanner and it works like a charm. Especially since I do not scan stuff often, so a program with more complex UI would have the effect that I forget how to use it until the next time.
I would start from the docs. There is pretty good reaper manual available from its homepage, covers everything from basic “how does one creates the prohect?” to relatively advanced topics.
Most of the plugins I encountered have no native Linux version, but yabridge deals with most of them quite good. I personaly use it paired with Play on Linux to logically separate different groups of plugins and everything works like a charm.