The nickel strings on a banjo might be an issue. With an arthritic hand I would go for softer nylons as a uke might offer. The fluke is a already mentioned and a good one. A soprano is nice, but the frets are really narrow spaced. A Tenor has a much wider space. It depends a little on how big your hand and arm is. You can also provide a tenor with a deep g-string without problems in intonation. Yes of course it is a serious instrument and a lots of fun.