Parent firm moved my company and 3 others to a new building, with no IT staff, and it's becoming my problem

Hey reddit,

A couple months ago, my company moved into a new space that is halfway through the remodeling process, at the behest of the investment firm that is our primary shareholder. The new space is going to house the investment firm, my company, and 2-3 other companies like ours that the firm "owns". We moved in first because we're the smallest (startup of under a dozen people), with the parent firm branch mostly moved in as well, and the other companies to follow in a few months when their part of the space is done with construction.

At the time we moved in, I (an idiot) inquired what the network situation would look like, since I was at the time looking into servers for our internal use, and so I (like a moron) offered to help another engineer run to Best Buy and rig up 4 routers as access points to get basic wifi up and running during this transition period. At present, my company and the investment firm employees are all sharing this one wifi network, which is mostly comprised of Linksys' out-of-the-box settings with a new SSID and password.

Turns out, there are no standing plans to get any kind of professional IT staff in-house. Today I overheard some talk between managers that leads me to believe that the network I set up needs some changes, and from this I intuit that soon I'm going to be asked to change passwords or configurations or some such.

The thing is, I'm a mechanical engineer. The extent of my IT knowledge comes from building my own gaming computers and hanging around with software engineers in college. I am at best a hobbyist-grade nerd. The current network setup is woefully inadequate with respect to security, my ability to administrate it, and likely bandwidth, if any of the other companies do anything network-intensive.

At some point in the near future I'm going to have to make a strong case for hiring professional IT staff, and I simply don't even know all that I don't know. How can I best make my case for why they should spend the money for a real IT professional? I want to point out all the things that could go wrong with multiple distinct companies all sharing a single consumer-grade network, but I'm not technically well-versed enough to think of all the ways that this could go wrong and the reasons this is a bad idea. Furthermore, this isn't my job - every hour I spend googling "how to change an IP address" is an hour I'm not doing the job I was hired to do, and so I don't want to frame it in a way that ends with "sounds like you need to do some homework" and it stays my problem.

What should I bring up in a future meeting to convince the higher-ups to shell out for an expert? How do I best explain to a non-technically-versed manager why the current setup is not acceptable for a building of 5 companies?