Accordions like ukuleles, have a tendency to breed. Just sayin'. Having been down that path myself, it's best to buy one that's fully functional. There are so many moving parts and so many ways for it to break down, it will probably cost more to repair. For that reason, eBay is not the best route. But one sometimes takes that risk and discovers" what they say," is true. Not that it happened to me. LOL
Following what Maki66 says, he just covered the two row gamut. There's the one row that's used for Cajun. The three row for Mexican music. That's just the diatonic variety. Diatonic - think harmonica. In and out produce different notes. Right hand plays the melody. Left hand has the chords. So you have to learn to coordinate both sides. Then there's your tone preference. Dry tuning versus wet tuning. I prefer wet where the tones mush together like in French accordion music. Dry tuning I think of sounds more like a piccolo. Piercing, high-pitched.
So the type of accordion you procure would have to do with the type of music you want to play. French, Irish, German, Acadian, British, Mexican and that's only a start. It's not exactly a self-taught deal so it would make sense to find a group you can play with and learn Tunes. I had a Hohner Corso I recently sold. I got it to play French music. Once the ukulele bit, and other circumstances arrived, I unloaded my accordions.