My 2c. You should pay a consulting engineer to oversee the work and approve progress claims from the builder and gather tenders and make recommendations. That will take a lot pressure off the SC. Normally CE charge by the hour or by a percentage of the total contract price. You need to work out which option will be more cost effective. Percentage based could cost more if the work and scope blow out. Take your time on large contracts and make sure everybody is in on the detail. Often its a good idea to break teh work down into smaller chunks. That way you can flick the builder and re-tender if needed.