Hi David,
You are about to dive into murky waters.
Wheels have a number of parameters:
1. diameter - usually expressed in inches
2. Width - usually expressed in inches
3. Stud Pitch circle diameter (PCD) can be expressed in mm or inches and has a number indicating the number of mounting points. For the MA Mondeo (read years 2007, 2008 and (I think) the early part of 2009) this figure is 5x108 ie five studs and a circle of 108 mm diameter.
4. Offset this is (usually) expressed in mm and can (usually is) a variable number and depends on the tyre width fitted to the rim. It is designed to ensure that the centre line of the tyre tread falls on (or very close to) the track width of the car.
5. Bore diameter; can be inches or mm. For the MA Mondeo it is 63.4 mm ( very, very close to 2.5 inches), to match the centre "nub" of the brake rotor so that the wheel is centralised with the stub axle. Small oversises are OK as the wheel nuts, when tightened, will centralise the wheel.
The MA Mondeo came with a number of wheels in various diameters. There were 16 (steel only and black with a wheel trim) 17 and 18 inch diameters. The latter two were alloy with five Y spokes.
A whole range of tyres could be fitted providing you altered the tyre aspect ratio so that the tyre diameter remained constant so that the speedometer provided a proper speed indication (whole other story about this).
If you go here;
https://www.wheel-size.com/ and paste in the appropriate data you will get a table of wheels and tyre sizes for the model. But be careful about the OEM bolding. Ford Oz specified other than the European norm for their imports..
Back in 2007 Ford owned Jaguar, Volvo and, I believe Aston Martin, and the MA Mondeo (Called a Mk 4 in Europe) platform was used for some Jaguars and Volvo models and so they used the same wheel specification , but different spoke designs unique to the particular brand.
Hopefully this gives you a starting point. As Mondaveo has said some late model Focus wheels will have the correct stud pattern and will fit in 16 17 and 18 inch diameters but they might not be the right offset (that ET number in the chart of the link) as the focus is a narrower track width compared with the Mondeo I believe.
Good luck, ask more questions for clarification.
Cheers