OK, £400 budget, to cover landscapes and wildlife with a DSLR. Let's have a swing at it.
It's pointless recommending any specific brand at this stage. All the manufacturers make good equipment. You'll see more Canon and Nikon stuff around because they share 80% of the market, so that's a good place to start if you're buying used. But they handle quite differently. Some people think Canons are intuitive and Nikon are all wrong; some see it the other way round. If you can play with some equipment before buying, that might help. Alternatively, pick the brand which more of your friends / family / colleagues use as that way you'll have a bigger support network during your learning process.
Do some research into the model ranges so you know what's what. DPReview is a good web site for camera reviews and specifications. Don't be too worried about older models, because (a) there won't be any compatibility issues with say 5-year-old DSLRs; (b) if looked after well they should be good for far more use than your average user will subject them to; and (c) you can make good big prints with any DSLR over 6 megapixels. For example the entry-level Canon range started with the 300D in about 2005, and went through 350D, 400D, 450D, and so on roughly a year apart until the current 700D. You'll see the specs evolve incrementally over the years. If you see a used 450D and a used 550D for sale it would be useful to know what the differences are and how much they're worth to you. Don't agonise too much about the decision though, because if you get bitten by the bug you'll be on a never ending upgrade path and it doesn't matter exactly where you start.
Moving onto lenses: these make more of a difference than cameras, so don't spend too much of your budget on the camera. Most DSLRs are originally sold with a "kit" lens, typically around 18-55mm focal length, which is good for general purpose photography including landscapes. Kit lenses aren't the best quality obviously, but some are better than others (for example Canon's kit lens with image stabilization is better optically than the older non-stabilized one, as well as being more usegul) so read and ask so you know what's what.
A kit lens isn't much use for wildlife though. Typically you'll want a zoom which goes up to 250mm or 300mm. There is lots of choice here, and to an extent you get what you pay for. For example Canon make three 70-300mm zooms which cost (when new) roughly £100, £400 and £1000. The middle one is the best value and is much better than the cheapest. Unfortunately the cheaper telephoto zooms tend to perform worst at the longest focal length which is exactly where you want the performance! All the manufacturers, including independents like Sigma and Tamron, make 70-300mm lenses. In addition Canon and Nikon make lenses designed for small-sensor DSLRs (Canon EF-S, Nikon DX) which are around 55-250mm and these tend to punch above their weight.
Alternatively, you could go for a single do-everything lens like a Sigma 18-250mm or a Tamron 18-270mm. Obviously a lens like this is a massive compromise and doesn't have the quality of a lens with a smaller zoom range, but it can be very convenient when you're starting out and don't really know what you'll find most useful. All your photos will have the focal length recorded in the metadata so you can easily look back to see which focal lengths you actually use in practice, and therefore where it's worth investing in upgrades.
Hope that helps for starters. Don't forget to buy a spare memory card and a spare battery, and maybe a bag. Some people will say a tripod is essential but cheap tripods aren't worth having so hold your fire until you get some personal experience. You'll need some basic photo editing software if you don't have any already. Read, ask, practice, learn. Recognise the symptoms of Gear Acquisition Syndrome. And enjoy!
