A 24mm is hard to beat; I've used one as a prime lens for more than 15 years now. Less popular is a lens going down to 17mm (or less) — this will require careful choice and composition given the extreme depth and ultra wide view.
Adding a 35mm or even a 50mm lens to your kit will set you up for a long time.
"Focus shift when stopping down"!? Never heard of it. Distortion can be common ultra-wide to normal zooms, less so in primes and almost undetectable in highly corrected (APO/ASPH) optics. I can remember using some early OM Zuiko and Tamron lenses in the early 1980s that had patently terrible distortion (pincushion, barrel, chromatic... just about every one of the Five Aberrations of Seidel!).