I can't say that I'm an accordion player by any stretch of the imagination, but I do own one. It's a 120 bass Titano and the most fun that I have with it is playing around with chords. 120 bass is a good option, but make sure that the piano keys (it's a piano accordion, right?) are full-sized rather than half-sized ladies'/kids' accordion keys (they shouldn't feel 'cramped'). I am also not sure what your accordion is worth (sorry to post and not answer your question

) but I think that your priority would be to find something that you can play. If that one doesn't suit your style or doesn't feel right, then trade or sell for another one. Otherwise, play on! It's also definitely worth looking into an electronic accordion with silent practice and multi-voice options. They really open up your options for practicing places.
Concerning actually playing the thing, one of the more useful tips that I have had is to use your middle finger for main note keys (on the bass side) and your ring finger for the main bass chords. That leaves your index finger free for scales. Secondly, it is suprisingly easy to create a good traditional "oom-pah" sound with bass keys with a simple I-IV-I-V7/I-IV-(I-IV)-I pattern alternating between your base note and the major chord. It's fun just to do that over and over. Thirdly, many comon chord progressions for pop/rock songs and even 80s music or ballads are kind of tricky to do because of the jumps, so it's sometimes helpful to approximate with closer keys. My final tip is that the bass side always overwhelms the piano side, so it's often hard to do any slow songs with sustained chords and still have the melody audible. An electronic accordion is also better for this reason because on some of them, you can actually adjust the balance of the bass vs the piano. Upbeat polka-ish songs are better because you can sort of 'jump' on the chords, alternating between either the base note or another note of the chord and the chord itself and creating the beat for the song. Otherwise, you may find yourself adapting a slow, depressing song in a minor key into something a little bit more...strange.
EDIT: I got the fingering thing completely backwards. The index finger is used for other chords while the middle and ring fingers are used for primary base notes (row 2) and secondary base notes (row 1) respectively
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