$300 isn't going to cut it unless you realize that the $300 is just an initial investment. The approximate amount of cash you will end up spending on a used DSLR camera set-up is going to be in the $450-$500 range since you will want more than one lens.

The D40 is cheap, and very prevalent but you can usually get better deals on the forgotten Canons (XS and XT cameras) if you hunt around on craigslist (there is less demand for those bodies).