40kHz Ultrasonic transducer - datasheet gives capacitive load at 1kHz what is it at 40kHz?
In the title you ask about 40 kHz but in the text you only mention 25 kHz. Since the transducers from the datasheet are 25 kHz transducers I will assume that 40 kHz was a typo and ignore it. A 25 kHz transducer would not work very well at 40 kHz. If you need 40 kHz, get a 40 kHz transducer.

Is this a reasonable assumption for designing an amplifier?

No, look at the plot of the impedance over frequency. Note how the impedance changes roughly a factor 10 within about 2 kHz. That's because such a transducer is also a mechanical resonator. It resonates at around 25 kHz which causes the steep changes in impedance around that frequency.

The capacitance of the transducer is stated at 1 kHz because that is "far away" from the 25 kHz it is supposed to be used at.

Here's an electrical model of a transducer:



At the low frequency of 1 kHz you'd only measure the value of \$C_e\$, the influence of \$L_m\$, \$C_m\$ and the \$R_m\$ resistors is so small at 1 kHz that we can ignore them. 

But at 25 kHz it is a different story! Then the circuit elements that we can ignore at 1 kHz now have a significant influence on the impedance of the transducer.

However, I think you're overthinking this as fortunately the impedance of a transducer is quite high (I see 1 kohm or more in the plots) near the resonance frequency so it is "easy" to design an amplifier / driver for that.

Start with looking up some circuits which are used to drive a transducer. Ask yourself why it is done like that.

Do you think that you will need a high current and/or high voltage to drive a transducer?

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