What is a sufficient way to ground my 2 layer low frequency PCB inside a housing?
Is it a good idea to pour the entire bottom layer as GND? Or it can be
  on the Top layer too?


It's usually a good idea to fully pour the bottom layer and, you can optionally try to flood the top layer too and connect it to the bottom full pour with vias every centimetre or inch (not critical on this design by the sounds of it).

I would also suggest you apply at least a 20 thou (mil) clearance to the flood so that it doesn't encroach to closely with the top copper tracks. Most PCB tools allow you to set this up.

Power tracks should be appropriately thick and it might be worth adding decoupling capacitors in a few places (your call because I can only guess what interconnections you have).

I'd also add a few test pads so that if needed, you can probe on various nodes.


  Should I connect the GND pour with the mounting holes?


I'd use an 0603 resistor to connect each mounting hole to the ground plane then you have the option of both scenarios. There usually isn't a best-answer that suits all scenarios on this question.

Add a silk screen with written details and rev number on and I think you are good to go.