Maintenance Overkill at 40K Miles?

Engine oil:

You're able to see the previous service records? When was the last oil change? I wouldn't change ahead of schedule. What you deem appropriate interval, is up to you. Personally I do 6 month or 5000 mile. What's it look like on the dipstick, level, appearance?

You'll need a 64.5 mm (inside, face-to-face) 14 flute socket for the oil filter. There's a plethora of "Toyota" oil filter sockets (aka "wrenches") being sold, that have extensions down onto the ribs of the Prius filter housing. I'm using the standard Honda automotive oil filter socket fwiw, works fine. Drain bolt is 27 ft/lb, and the filter housing is 18 ft/lb. Oil quantity to put the oil on the top mark (with filter change) is 4.5 quarts I believe, 4.2 liters up here.

Transaxle fluid:

Verify from the previous service records, but if it's been serviced by the book it's likely never been done, and is a good idea to do, sooner than later. Use nothing but Toyota ATF WS to play it safe, 4 quarts is more than enough. Level is right when it starts flowing back out the fill hole, with the car level. A funnel with hose from above is the simplest method, and use it only for that fluid. Dealership sell the fluid, for under $10 per quart, and also has the fill and drain bolt washers (they're identical). Both bolts are cap screw style, with 10 mm hex recess. You can buy "sockets" with the allen key incorporated. Torque is 29 ft/lb.

Coolant (2 circuits, engine and transaxle):

Way too early.

Engine and cabin air filters:

Just check them for starters.

Other items to think about, check:

Brakes.

Simple stuff:

Tire pressures and condition. Including spare.

 

I like your idea as this give you the new owner a clean starting baseline.
Don't forget the brake fluid as it is hydroscopic.

The air and cabin are cheap and easy and never hurts, yet you could check their condition to see if they were recently replaced.

The engine coolant though (and second circuit coolant as mentioned above).
Check the manual.
Many vehicles 5 year coolant, so this could be premature and a waste.
Post # 3 also offers some valid advise, including buying the correct oil filter wrench.
Yet, as a hobby, new coolant is awesome.

Have fun.