The German king, as noted, is probably the least objectionable part. If the Poles get as much choice as they expect, and are smart about it, they'll pick a young, unmarried scion, and quickly marry him off to some respectable Polish woman; this ensures that at the very least, this binds him more closely to Poland; and any child he sires will grow up entirely in a Polish environment (whereas if, say, the candidate had a ten year old son and heir, said child would already think of himself as primarily German). Catholic goes without saying, as well, a dynast with an interest in Polish culture (preferably already with some Polish language ability) is preferable. Other conditions should apply; to accelerate assimilation; the language of the court will be Polish, not German; his heir will attend a Polish school, not be educated by tutors, and just in case; stick in as many constitutional monarchy clauses as possible. After a generation, the new dynasty will be as Polish as Saxe-Coburg-Gotha is British. Of course, the biggest issue with this is that it just feels remarkably objectionable; to plop a German butt on a Polish throne.