Secure LAN within existing office LAN

First things first, I was gonna post this on Server Fault but honestly, I'm no network admin, I'm a CS student who has been called upon to sort something out for a very small family business who have just moved into a small office space and don't really have the cash on hand to hire someone in to sort it so I'm having to lea what is needed to complete the job. I'm also aware that this question 'LAN within a LAN' has been asked before so feel free to mark this as a duplicate though none of the existing questions really answered the questions I have.

Thus, the issue. The office we've moved into is being converted from a large building previously used by a single business into a 'business centre' with individual rooms being rented out. Each room is wired up with several etheet ports leading back to a network room with a cabinet full of switches to tie everything together although none of that is in use as far as I can tell. The guy who managed the network was made redundant and it's now mostly a shrine to his lack of cable management.

The current businesses that are occupying the rooms are all relying on a wifi network provided by a 'BT HomeHub' ISP provided home router/modem combo. As we're Govement regulated, I don't like the idea of sharing a network and I doubt the regulators would either.

So, what are the options here? I can't really do anything about the home router/modem as there are multiple other businesses sharing this for wireless access. I'd ideally like to access the inteet via this modem but need to ensure that the network we're running on is completely inaccessible by the other devices on the network that aren't part of our business. I've been browsing some of the small business router offerings by Cisco along with wireless access points (wireless access being the immediate priority) but I'm not sure if I can achieve the above with one and want to be certain before I order any hardware.

I'm sure the best option would be to simply run another line into the building but that adds an extra monthly cost plus a service contract so I'm keen to avoid that for the moment.

Any thoughts on the best option in this situation and how I might go about it?

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