Are they in the SERPs what so ever? They may have been hit by panda or penguin, how about any of the other rankings have they changed ? Have you checked outside of Moz to confirm ?

If you go by the webmaster guidelines you can't go far wrong.

It could be they were up to some shady practices and got penalized for as such.

Where do you want to get most of your traffic from? If it's from the say the UK i'd say you should go with the .co.uk domain (plus you can redirect the .com to any visitors can still find you better).

Obviously there is a bit of a set up involved but I think you gain access to those local searches which can help. Still worth researching all options

Best of luck what ever you go with.