Before starting a project, you must plan for it. You do need to hire planning specialists for a one-time job. You can Easily assign it to PMSP who can provide you on Task per Task basis with the following:
Project Budget Analysis
Project Cash Flow Analysis
Documentation Management System
Any of these tasks could be assigned separately while the rest is to be executed by the in-house team whenever available. Hence, you will be able to control and limit the cost of the planning stage by executing whatever available via the existing in-house team, and assigning the rest to the Specialists at the most economic cost of service.