There are two different systems. On the later cars there was the option of the Tyre Pressure Monitoring System (TPMS) which involves special valves with a sensor inside the wheel and a monitor unit in the engine compartment which communicates with the wheel sensors by wi-fi/bluetooth, or something. This is not a realistic aftermarket fit. The system beeps and puts warning messages on the dashboard if the pressure drops, but you can't see the actual pressure. That's only available with a Tech2.
The "Tyrepal" system is aftermarket only. The one which I have on the 9-5, and the Carlsson winter wheels, uses sensors built into replacement valve caps that fit outside the car. I've used them for several years now - at least 10 years - and they work well. There's a small unit that sits on the dashboard that shows the pressures, and you can set a low and high pressure value at which the system starts flashing and beeping as a warning. It can also warn against high temperatures.
Both systems will also warn if the pressure drops quickly.
This is the latest Tyrepal system. The basic system comes with four valve sensors. You can get extra sensors for the spare wheel and also for a trailer or caravan.
https://www.tyrepal.co.uk/product/kit-brand-new-solar-colour-pro-monitor-4-tcso-sensors/There are frequent offers around which bring the price below £100.