Technical Advisor. Do I need one?

You only need one if you are a business man/woman, an enterprise or a company who takes their investement seriously and responsibly. The one man show is not applicable in construction. There are a lot of reasons the previous statement is true even if one is capable of doing so:

Project feasibility

Feasibilities studies are undertaken in the early stage of design. It questions all the factors - economic, technological, legal and scheduling factors - affecting the project. The business case of the project needs to satisfy the clients' needs and all potential problems are identified.

Design hypothesis

There is a series of hypothesis made during the design stage that very often fall into review when the construction starts. This is a standard practice, otherwise you would need to pay a lot more for designing to eliminate those unknown factors.

Review of Design

Even if you have chosen the best out of the best designers, their design needs to be reviewed. Every professional team of architects and engineers will go for that and participate in a review that will eliminate or/and validate their design. Futher improvement? Why not?