it should be deep enough for the turtle. Now, talking koi, can we talk? Koi can get over 30" and are a huge bioload all unto themselves, not to mention your turtle. The general rule of thumb is 1000 gallons for the first koi and 250-500 for each additional. Koi can be fine in 24" but my recommendation is 3' minimum, esp if you get winter proper with ice and snow for any length of time. Do read up on how turtles brumate and have UW structures for them to wedge under/into or a container of 6" sand and clay.
btw, there is no real 'standard depth'...
I figure you'll be near 1000 gallons when you get done, so how about considering goldfish? Shubunkins are one type, they don't get as large as koi, and have a multitude of patterns/colors. The bioload from gf is a lot less and you don't need to be as concerned about feeding them.
All that said, by all means consider koi, but with a larger pond in mind. Too, the filtering needs for koi are greater...
As you allude, turtles are escape artists, so you'll defintely have to fence them in. For most ponds, I'd suggest you plan on fencing it in as opposed to the yard, but if you go around the perimeter and guarantee no small holes large enough for your turtle to scoot through, that's all you need. Digging, now, to stop this, what I did was to bury some plastic chicken wire 12" below ground level. This will stop that behavior. Too, what I found is that turtles will climb out and seek escape even though you sit them down and tell them the rules. They aren't great at listening and will still make the attempt. I've had turtles get stuck between rocks around my pond, necessitating rescue. Even lost one when he got stuck between rocks on my waterfall and I didn't even think to look there. Morale of this story is; think/loo/plan to NOT have such turtle traps. If you think they can slide/fall/climb and get stuck, prevent it by closing up gaps with smaller stone and or soil.
Welcome to GPF!
And if anyone asks, you DO like pineapple on pizza! Don't ask, and don't let the others dissuade you...!