It is safe to say that rules for "all" anything in Polish language are rare.
Feminine nouns end in singular genitive with -i or -y. That is also true for -a ending masculine nouns.
you can take a look on this table https://pl.wiktionary.org/wiki/Aneks:Język_polski_-_deklinacja.
masculine nouns end with -a, -u, neuter nouns end with -a
Also neuter and feminine nouns often have singular genitive=plural nominative. (Lampy)
You should think of things in terms of declensions not genders. This way you will see that most polish nouns are, in fact, regular https://en.wikipedia.org/wiki/Polish_morphology#Nouns
If you look under kupić, though, you can find a derivation from Germanic kaupōn/kaupjan. So it is indeed related to Dutch kopen, German kaufen, and even English "cheap." These in turn come from Latin caupo, "shopkeeper."
Szukać has a less certain etymology, but according to the dictionary, the usual explanation is indeed that it was also borrowed from Germanic: East Germanic (Gothic) suochen.
Proto-Indo-European -> Proto-Germanic -> ... -> Dutch. Proto-Indo-European -> Proto-Balto-Slavic -> Proto-Slavic -> ... -> Polish. Some words in Germanic and Slavic languages definitely have common origins, for example my in English, mein in German and moje in Serbian (also water, wasser, voda; thou, du, ti; bottle, boca; cat, katze, kot; mother, mutter, mater; ...). If you're interested in this topic, look up Indo-European on Google. Langfocus has a good video called "Indo-European connection".