To perform the most correct and realistic valuation possible, it is necessary to start from data that reflects the economic reality of the company as best as possible.
These insights can be obtained by the following information:
Annual accounts
In principle, this information is always available,but it is usually not sufficient to make a full valuation
Trial balance and analytical accounting
The more detailed trial balance provides a more in-depth insight into the various cost and income accounts. It is therefore very useful for the preparation of a valuation.
If there are different product lines or business areas, analytical accounting is a very valuable asset.
Management reporting
Interim reporting can also be an additional asset in the fine-tuning of the figures. 
Business plan
The more insight you can provide in the future, the more substantiated the valuation exercise is, since it is always based on a number of assumptions.
List of non-recurring income and expenses
Having a good grasp of one-off costs or income is also very important to have a correct picture of the economic reality of the company without impact of these non-recurring elements.
Since these revenues or costs can sometimes have a very large impact on the valuation of your company, this reflection is essential. 
Strong and weak points
Consider the strengths and weaknesses of your company, but view this from the perspective of a potential acquirer