The point about fixed rate versus hourly depends on the value of the estate and the complexity of the work. Generally, it would benefit the solicitor to charge a % for a high value estate where work straighforward and non-contentious, and an hourly rate for a low value estate.
A practitioner familiar with this area of work would have streamlined his/her workstream.
The issue is not fixed rate versus % but whether a fixed price is available in the market. In a competitive market a fixed rate should be sought in order to benefit from any efficiencies which are available (hourly rate does not give this incentive). Obviously, lowest price may not always be best in terms of service scope and quality.