$1000 is a pretty good sweet spot for value. You should be able to find a bike with the following:

1) Air fork w/ through axle
2) Hydraulic discs
3) 9 or 10-speed drivetrain

As for 27.5 or 29er. I'd say it goes with sizing. If you're 6' or above you'll probably like the larger 29ers. If you ride small frames, stick to 27.5. If you're in the middle, it can go either way.

For example this Fuji 1.5 is a lot of bike for $900
Fuji Tahoe 1.5 29er Mountain Bike - 2016