Honestly, your better off buying new.
Second hand comes with a lot of issues from sensor issues through to motor issues for the shutter.
For around the £300 mark you can pick up a starter DSLR such as the Nikon D3300 or Canon 1200D. Both are a good way into the world of SLR photography.
A very simple way to look at things is that you will get better photos from a cheap body with a good lens than you will from a good body with a poor lens. Lens quality is everything when it comes to good clean sharp photos.
Current "Budget" DSLR's are in fact better than say 5 year old DSLR's that are now selling for around the £200-£300 mark. My camera body cost £900 when released a few years back, yet a modern £400 body is ahead in pretty much all tests.