Sorry you are going through this.
Are you married? If not, then, as far as I am aware, his capital assets such as property, savings, etc., are not taken into account. If you can come to a private arrangement, then great, but, otherwise (via the CSA), it is not about what is 'reasonable', or how old the child is or what they need, but is just worked out on a flat percentage of his income.
As jelly says, via the CSA (if they do their job!), you would receive 15% of his net (after tax) income. Depending on your earnings (based on last tax year), you would also get child and working tax credits which can be a big help. Visit www.entitledto.com which has an online calculator to try and work out what you would receive. If you have a low income AND receive very little child maintenance money, then you might also be entitled to help with council tax and/or rent (not sure about rent though, perhaps someone else knows?) It might be helpful to visit the Citizens Advice Bureau or speak to the tax credit helpline.
In your place, I would work out what I would get via the CSA first and then this as a basis for negotiation with your ex. It also depends on how amicable it all is between you. You might prefer to go via the CSA in order to not have to negotiate with your ex but he might resent this and the CSA might take ages to sort things out which would leave you worse off than if you had come to an agreement with him.
It would be a shame to sell your flat. You may find, once you have worked out all of the above, that you and dd can afford to live there yourselves.