The scenario I have chosen to manage is the training room evnvrionment upgrade. I have seen, and been a part of, both sides of this scenario. There is a lot that can be said here, but I will try and keep it simple. Hiring a vendor helps relieve the stress from the manager by having the vendor perform a site survey, recommending necessary equipment and cabling to provide a stable and current infrastructure that will last for years to come and require less maintenace. However, this comes at a cost.the vendor will need to fly folks on site, rent hotel rooms, pay to actually perform the survey, and even be charged for the time to put together a scope of work and SWOT analysis. Then there are labor costs, time management, site availability for the work to be completed. Every detail and time must be tracked and paid for. This can become costly, but worth it.
If the work is to be done in-house it can save money, but could have other problems. I the staff is not qualified or has never performed a site survey they could have a plan in place to move forward, but could be missing key information. Cable run lengths, type of cable, how to connect the infrastructure, what equipment is needed to where it is not overkill or it is not enough. There can be wireless deadspots throughout the area because of an unreliable site survey. Most inmportanlty the SWOT analysis could have missed key threats or oppurtunities to improve because the staff does not have the experience in this endeavor to try and mitigate any and/or all possbilities that could harm the overall project completion and lifespan.
If the money is available. I would go with an outside vendor unless I was 110% sure that my IT staff could accomplish this goal with proven experience.