When considering going digital with a DSLR, Chris, where to start is a simple question but all the answers soon get complex. I have found this review and advice site useful
http://www.imaging-resource.com/ but I'm afraid your budget is a bit tight if you want to get into serious digital photography, even secondhand.
Now for some very rough suggestions: Some people like the simpler option of what is sometimes called a 'bridge' camera which is basically an advanced 'point & shoot' with just one fixed lens. They are relatively inexpensive yet have most of the controls and settings of a full DSLR. When used correctly, they can produce some good work and make a cheap learning camera.
If, however, you want fully interchangeable lenses it becomes a bit more of a minefield. Firstly choose your company (Canon, Nikon, Olympus, etc) but choose carefully as you will be 'selling your soul' to that company for evermore. Or face an expensive change of alligence. In reality there isn't a lot of difference between them as regards quality at the same price level.
If you can find a good secondhand camera at a good price that may be sufficient to make up your mind. That is what happened with me. I bought a Canon 10D plus lens on e bay; but too late I found that it was a 'dropper' or some similar catastrophe happened to the lens which had developed some strange faults. However, I eventually purchased some better lenses but have become a 'Canon man' now.
I know some people who have had good deals on e bay but that was my one and only purchase and I would never feel safe there again.
If possible I would suggest going to a shop and handling a few cameras to see what feels right in your hand, even if you don't purchase anything there.
As a very rough idea, and just going with Canon for now. The 400/450D are good cameras with full DSLR settings but unless you get an exceptionally good deal will be outside your price range. You may find a 350D secondhand at a better price and apart from fewer pixels it isn't greatly different from the newer models.
Personally, I prefer a larger and heavier camera so I currently use a Canon 40D, but you could find a good deal on a 20/30D secondhand.
Nikon have similar alternatives but I will the details on those to a 'Nikon man'.
All in all it is difficult to advise you until you have made the 'brand leap'. Although I would mention that, on reflection, it may make some sense to simply save a little bit longer so you can join the 'camera arms race' at a slightly higher level, which could work out cheaper in the long run.