Hourly billing calculates charges by reference to time spent at agreed rates. An estimate, a cap and a fixed fee are different arrangements; the written engagement should explain which one applies.
Check who may work on the matter, which rates apply and how time and expenses are recorded. Ask how you will be told about progress against an estimate and how additional work is approved.
If a cap is proposed, read its scope and exceptions. If the proposed work changes, clarify the effect on the charging arrangement before that work starts.
Our scope and fee guide sets out questions to discuss before engaging a business-sale lawyer.