Any. Really, this is the good news! Any DSLR is good enough to learn photography. They all have manual controls including manual focusing, custom white-balance and shoot both JPEG images and RAW files. Sure, some are better than other but this is all you need to take control of the photographic process and will give you plenty to learn.

There are two features which are worth specifically looking for which are available in a good number of entry-level DSLRs, but not all of them:

• Spot-Metering: Lets the camera meter from a small zone in the scene which is great to have when learning to expose creatively and learn about things like the zone system.
• Depth-Of-Field Preview: Lets the camera show you before the shot an approximation of what the depth-of-field of the final image will be like.

You budget is obviously a limiting factor when you include the lens, so while you will be able to get a generic zoom, I suggest you go for a bright prime lens such as a 50mm F/1.8 (even new it's only $120 or so) or wider 35mm F/2 (or similar depending on the brand). This is more restrictive for framing but the bright aperture lets you play much more with depth-of-field and lets you work in lower light. Not only that, cameras focus faster with a bright lens.