Suggestions on Wifi Hotspot Management with SMS OTP Authentication wanted

Hi

Newb network 'guy' here, please excuse the rookie questions, I hope this is the right place to ask!

An intending client approached me to propose for wifi hotspot management where they want to collect data from intending internet users and which will send an OTP by SMS to the user's phone for authentication. User data must be collected and sent to the client.

Wifi equipment, and internet access to be provided by them, I can suggest, but I'm really only supposed to design and install the hotspot management software, networking and SMS OTP parts.

Im looking for any suggestions on I can configure and deploy, that can come in cheaply enough?
Im not averse to a bit of work (I was investigating a combination of managed switch, pfsense+packetfenceif it comes to that).
Any advice greatly appreciated.