The longer barrels are because of the open sights fitted. Your BRNO is super long exactly for this reason and you could cut it to 12" with no discernable difference in performance or accuracy (I chopped my BRNO to 14" and it was fantastic, wish I still had it). The 20" barrels are a throwback from the open sight length, nothing to do with performance.
You've answered all your own questions there Stubby. Go second hand, you won't be disappointed. Trigger done, bolt worked in, barrel floated, barrel fouled etc, etc. Also nine times out of ten you will get scope, mod, mounts, sling,