You'd still need to keep the indoor setup for winters unless you're in a VERY warm area (and even here in central FL I have 3 heaters in the pond for 'cold' snaps)....and also in case one turtle needs to be separated due to aggression or medical issues.
I agree though...my pond is much easier maintenance-wise. That said, it's suprising how much work you DO have as soon as you add turtles to a pond. Do you have water plants you care about in the pond? Uh-uh. They WILL become food (or confetti....) The koi don't make NEAR the mess the turtles will, and you'll still be cleaning out the filter on the pond too. In fact, you may need to upgrade the pond's filter just to keep up with them...I've smelled a stinky pond, and believe me you DON'T want one, LOL! Also, when you consider fencing, don't forget that turtles are wonderful climbers - AND DIGGERS. You'll need to have fencing not only twice the height of your
grown turtles, but also have to go underground about a foot unless you want to find that they've tunneled out one day. At my pond I see cats, possums and egrets all looking for a meal now & then. Until I put the pond in I'd NEVER seen any of these animals around the house. You can't assume that your lawn is a safe-zone.
I used a plastic chicken-wire-type fencing from Home Depot, dug a trench around the pond (leave PLENTY of land area inside in case you have females that will require nesting grounds!), sunk the fencing (curved it in a few inches), and filled up the trench. Then I used wooden stakes to hold the above-ground portion in place (it's a rounded area I have fenced). Soon I'll have to add another roll of fencing & attach it all above ground because my turts are growing and the fencing is no longer tall enough if they wanted to climb. Honestly there HAS to be a better way to enclose it - that was the roughest part of installing the pond. I had to go the cheapest route though, kwim?
How big is your pond? You still have to consider the 10gal per inch rule of thumb...and you also have to think about extra room for the koi. You're looking at 400+ gal minimum I'd think.
That said...my turtle pond is my absolute favorite place to be around the house. I'll take a book out & sit...and end up watching the critters more than reading.

I'd recommend a turtle pond to anyone ...as long as they don't think it will be low maintenance