The 7th gen Nano can still be found in good condition, but the better examples tend to be a little pricey. A used iPod's battery condition is always an unknown, and replacing the 7th gen battery requires delicate work and a very small soldering iron (see iFixit for details). There are "refurbished" ones for sale on eBay, and even new ones (for stupid-high prices).
I believe that the 7th gen Nano was the last iPod sold, aside from the iPod touch. The bluetooth works well, although using it drains the battery a bit faster, as does the FM receiver (you have to use wired earbuds for radio, the cord is the antenna). It has 16 GB of storage.
For the gym, I favor my 2nd gen Nano. Its 2 GB of storage is more than enough for multiple workout playlists, and I don't worry so much about sweating all over it or losing it. Same for the cheap wired earbuds I use. It also looks grungy enough to probably be safe from theft