Based on my experience, I would suggest that once mediation starts you begin discussing child arrangements first. This way if she keeps bringing money into the discussion the mediator will soon pick up on this. Just like my ex wife, she will no doubt say that she wants the child to stay with her full time, you need to counter act this and say exactly the same thing. Even if you know realistically you having the child full time wouldn't be ideal, at least this way when you start saying 50:50 or 60:40 to her you will be seen as the reasonable one who is trying to meet half way.
Go in there prepared, note down all the advantages of the child being with you. A few points I used were that I worked close to my sons school so collecting him would be straight forward, I take him swimming lessons every Thursday, he has a close relationship with my family who all live close by and that he could become isolated if he was to live with him mother full time and so on. My ex wife did no preparation regarding our son and was only thinking about her claim to money, she was caught of guard so the child arrangements were drawn up pretty quickly.
Once child arrangements are agreed, of course this will put you into a better position regarding the finances.
There was a programme on the BBC called
Mr v Mrs: Call the Mediator.
If you have time, look it up on the internet/bbc iplayer.
It might be worth watching before going to mediation. I think it's really insightful and shows how each person's body language, tone and attitude influence how productive the sessions are. It also shows you the role of the mediator in the process.