Legal analysis of the earlier relationship, retrieval of records held outside the employer's own systems, and possibly translation of contracts or tax documents from the relevant jurisdiction. These sit on top of ordinary preparation and should be scoped separately so the employer can see what the history question costs.
Fund the analysis before the filing
This is a case where an early opinion is worth paying for on its own, because the answer determines whether there is a case to prepare now or in some months. Ask for that assessment to be quoted separately from the full preparation. Record retrieval from a former intermediary can also carry costs and delays that are easier to accept when budgeted rather than discovered.
Ask for the early opinion to be quoted with a stated scope and a stated output, since its value lies in producing a date rather than a view. An assessment that concludes the qualifying period completes in a named month gives the business something to plan cover around; one that concludes the position is arguable gives it nothing. Where retrieval from a former intermediary is likely to carry a charge or to require legal correspondence, ask for that to be estimated separately, because it is the element most likely to exceed expectations and the easiest to approve in advance rather than mid-process.