Clarify what evidence already exists and what explanation is needed. A consultant can help organize facts, but cannot supply knowledge the employee did not possess.
Pay for accurate explanation rather than invented complexity
Separate evidence preparation from any commercial accounting or tax advice and from current government fees. The application should describe the real process, even if a simpler account is less expensive to prepare. Ask what each engagement is actually buying before agreeing to any of it.
Documenting a method that exists but was never written down is money well spent, because the resulting note is contemporaneous evidence of how the organisation works and it survives long after the petition. Designing a method that did not previously exist buys something quite different: a new process, whose novelty an examiner may reasonably ask about, and which the applicant cannot have spent a qualifying year operating. Hypothetical example: an outside firm is engaged to rebuild an undocumented allocation approach so that it looks rigorous on paper, and the finished model is more elaborate than anything the applicant ever ran.
The file now describes work that did not happen. Keep tax and accounting advice, evidence preparation and official filing charges in three separate quotations, and remember that the honest, simpler account is the one the records will support.