Is a 2A transformer too much for a power supply that outputs 13V to 15V DC
A home work question! But you declare it as such, and you already understand the concept - in this scenario (a voltage source) current isn't "pushed", it's "pulled" by the load. So the short answer here is that you'll be OK buying the 25 V, 2 A transformer.

A different way of thinking about this is for both voltage and current, the listed rating is what is/can be supplied. Therefore a 25 V, 2 A transformer will have 25 VAC on the output and the maximum current which can be drawn is 2 A. If the load draws less than 2 A, the no problems. More current is beyond what can be supplied, which will mean the transformer will overheat, with the follow-on consequences you might imagine.

You can't use the 12 V transformer because - assuming you're using a linear regulator - you need to supply a higher voltage to the regulator than what it regulates to. Exactly how much higher is a question for reviewing the data sheet for the device.