Craig is correct — the DSLR and lens should be considered separately. If you do find a bundle that has both you want then, of course, you can buy it, but first decide which cameras and lenses would be satisfying.

As I am mostly familiar with every lineup, I'll speak from that perspective ;)

Going used is a great opportunity to find excellent values. You may be missing low-light performance or the ultimate gimmicks — I mean features — like video capture and live-view, but you'll get more of what matters.

Every brand has a low-end lineup and one or more high-end lineups. You should really go for the higher ones, because they are built to be more durable, and so have a better chance of staying in good shape and having shutter-cycles left. Usually, the are rated to take between 3× and 10× more photos. As a bonus, you will get better ergonomics, more direct controls and larger viewfinders.

If you go with Canon, for example, an XXD camera like the 30D or 40D falls into this group. For Nikon, that would be the D70, D80, D90 or any Dxxx, like the D200.

Since you are considering telephoto and nature photography I would actually go with Pentax, Sony, or Konica-Minolta (acquired by Sony). The advantage is that these cameras have in-body stabilization which will save you money on the lens since you do not have to pay for in-lens stabilization.

You may be able to find the Konica-Minolta 7D which is built like a tank, the Sony A700, the Pentax K10D or K20D which are great. These last two Pentax cameras are weather-sealed as an extra bonus. A quick search on eBay shows the K10D for under $300 and the A700 for just over.

Then, you can find a compatible lens. On the cheap, you have to compromise on image quality and speed, so I suspect you will want to go for a 70-300mm or 75-300mm of the respective brand or a third-partly like Sigma. I see a Minolta 75-300 for Sony or KM cameras for under $100 on eBay. Most other brands go for a similar price.