If you ran Memtest86 for the standard workload and it didn't report any errors, that more or less rules out memory issues (even if they can always crop up, and absence of evidence isn't evidence of absence). An SSD can't really be stress tested in the same way - for a heavy-ish load, run any disk benchmark on it (CrystalDiskMark, for example), but none of those will run for particularly long, and none will show whether drive activity causes slowdowns in other applications. SSDs generally aren't unstable or cause system crashes either, unless the drive is faulty, in which case the only way to tell is typically to try with another drive. As for the GPU, Unigine's benchmarks are pretty good (Superposition for current-gen hardware, though Valley and Heaven is also decent if much lighter), though neither of these are interactive (no benchmarks are), so you aren't likely to be able to tell if the system becomes noticeably more laggy while they're running.
But as I said