All these answers are good advice. I too also play other instruments (mainly guitar these days), and I love the sound of free reed instruments in various styles of music. Perhaps a browse on youtube with accordion, melodeon, or concertina in the search box would give you some idea of the range of styles and sounds. Lots of Tex-Mex and also country music bands use accordions and other squeezeboxes, but there are also more traditional Morris players using melodeons and concertinas. As a guitarist, you will also be able to figure out chords for song accompaniments fairly quickly. The main thing is to choose a type of instrument you can afford and like the sound of, and then you will start to use it just like an extra stringed instrument! Enjoy : )