What should my first adult bike have?
£ so if you are in the UK, check out Edinburgh bike coop if there's one near you.

Definitely get disc brakes. Opinions on full suspension bikes differ - it used to be said that if your are spending less than $1000 don't get it. But I don't do extreme downhill so don't really know.

Most mountain bikers use camelbacks rather than water bottles because off road the bottles get covered in dirt (=cow shit) and you catch lurgy and die.  You can always add a water bottle holder later.