When I was starting out I went out and bought a secondhand Rockhopper off ebay.
It was in fantastic condition,almost unused.It was too big but through riding it over a year I got a feel of what I liked and what I didn`t.
I sold it a year later for pretty much what I paid for it and bought a Giant XTC which I still have.
The point Im trying to make is that,there is no definitive answer to bike bike buying.I tried loads before I bought the XTC,but although it suited me then isn
t necessarily what I`d buy now.
Go try as many as you can,buy on what feels right to you.On an £800 bike try to get good forks but don`t get to hung up on the intricacies of which groupset it has,but concentrte on the geometry,and how it feels.
I used to end up going round and round in circles ,confused by the number of choices.The important thing is just to get out and ride!