Let's offroad / mountain bike / mtb / ride dirt

First time mtb purchase approaching and I want some help and advice. I don't have any grand plans - definitely not racing - just a bit of rolling about on the downs/peaks etc. Budget is £600 and this is what I've narrowed down to:

1. Ex-hire Genesis IO ID (alfine hub one)
   
2. Brand new Haro Mary XC 29er
   
3. 2nd hand Orange Crush
   
4. 2nd hand Genesis Core 40/50 (seen both going for budget or less)
   
   

Open to other suggestions/offers! Cheers.

How tall are you?

I happen to know a man with a mint condition Cannondale Rush if you are in the market for such a thing.

You'd need to be 6 foot though.