Start with a defined repair estimate
Ask what the workshop has found, what the recommended work includes and what remains uncertain. Separate required work from optional or later items. If diagnosis is still needed, keep that cost and decision separate from a repair figure that has not yet been confirmed.
Label the value you are comparing
A dealer's advertised retail price, a private-sale estimate and a part-exchange or trade figure are different measures. Record the source, date, vehicle specification, mileage and condition assumptions. An asking price is not evidence that another vehicle sold for that amount, and a guide value is not a purchase offer.
Consider the replacement as well
Use actual figures for a vehicle you would realistically buy, any inspection or initial work, and the practical disruption of changing cars. Check its history and condition instead of assuming that replacing your current car removes maintenance risk. Keep unknown costs visibly unknown rather than filling them with a generic percentage.
Make the decision with the missing facts visible
Ask the team to explain the proposed repair and any assessment limitations. Use an independent sourced valuation where available and a separate appraisal if you need help discussing a replacement's condition. The workshop enquiry does not commit you to a repair.