The speedo can be off due to slight variations in the tires
diameter. The speedo counts the revolutions of the tire so it needs to
know the tire's circumfrence. (Yes, it varies a bit as tires wear
but a speedo is accurate to, say, 5% anyway). The dealer can reprogram
it via a software interface to compensate for any variation in tire size
over the tires it came with from the factory.

As far as the original question about problems: I'd say no. The
brake components should work with any tire/wheel Volvo sells.

The tires from a 2.4T might last longer. They are probably a "touring"
tire where as the 17" where probably a "performance" tire. My 98 T5
needed new tires at 24K miles; my wifes GLT wagon (with "touring"
type tires) made it to 33K miles. If you live where it snows, the
narrower tire will also work better in snow.

Since ultimate performance is not your goal I'd pick the wheels based
on what looks best and gives the ride you want. Sounds like you have
the correct choice.