I think I would go for the used bike option too!
Not really in the spirit of things though, is it, so if I had to buy new, it would depend on where the bike was going to be used.
If it was for my Pennine rides, I'd want my usual wide range of gears so it would be something like
this Felt from Wiggle.
For a flatter area, I'd go for a singlespeed bike and I'd be tempted to build one up myself. I could make something really nice for £500.