Outdoor toilet, office conversion

[ColdOutside]

Hi,

 

I have an outdoor toilet with double skin wall and cavity, it has tiles directly on concrete floor and used to have a door. Attached was a outhouse shed which I've converted into an office and then cut through into the toilet and blocked up the old toilet door. 

 

The office has it's own door too, so now there is a cold damp toilet and separate office which is fully insulated and warm (thus far).

 

That's on the context.

 

The issue is the toilet has dirt up the side where it meets party wall and after rain it's ingressing the bottom per picture.

 

1. Is it worth putting a water proof paint on there walls? As I cannot fix the source of the issue, so it's just a case of preventing the ingress.

 

I put in a new (cheap) sink stand as there wasn't a sink before (attached where door was), as this is sat on the cold floor and there is no heating in the toilet, it's covered in mould already, however the wooden shelves that have always been in there are not mouldy (above toilet).

 

2. Is this a wood/cheap issue? Where it's just got a high moisture content? Or am I supposed to be putting a heat source in there? or dehumidifier? I have insulated the old door and insulated the roof.

 

Thanks