I think any entry level DSLR are good cameras for learning. And most of them are able to take good photos including those older models. Keep in mind that cameras life cycle is rather short when compared with the lenses. Lower end consumer grade DSLR only have 1 or 2 years life cycle. For the Canon XXXD or Rebel series, new model comes out every years. So those few years old cameras are still very capable.
In general, you will find more used Canon and Nikon cameras because of their popularity. And for the same reason, more used lenses and accessories are available on the market as well. When a 3rd party lens company make a lens for a DSLR, I am pretty sure they will make one for Canon and one for Nikon. And since Sony is getting popular as well, so some will also be available in the Sony mount.
And the mentioned 1100D is not bad for starter. It has limitations, but by the time you know about the limitations, you may already know how to work around them or that is the time for upgrading the camera body.