Short answer: For an accurate quote, send the current manuscript or representative sample, word count, research field, service needed, target journal if known, author instructions, deadline, and any reviewer comments or supplementary files that are part of the job.
If you need journal selection, include your abstract, keywords, article type, indexing requirements, and any university or funder rules. If you need resubmission support, include the decision letter and reviewer comments.
If confidentiality is important, ask how your files are handled before sending unpublished material. Only send the information needed for scoping and avoid unnecessary personal or sensitive data.
A precise scope protects both sides: you can compare quotes on the same deliverables and the editor can give a realistic deadline.