Well Grant, my koi have just finished their 4 winter inside...for that matter, their 4th year inside. Due to business relocations I had to forgo havng an outdoor, ingorund pond and instead built a modular/portable, 2500 gallon system that some have referred to as a "cardboard box".
It has a 4" diffuser bottom drain (pond sits on 2x8 joists) that feeds a Nexus 200 and return to 2 TPRs (4 originally). It has a 4" gravity fed skimmer that feeds a vortex and then pumped to a Challenger bead filter. This returns to a wier water fall with a portion diverted to a DIY foam fractionator. When it Memphis it was in the garage with south facing windows. Here in Illinois it is in the basement with a couple of windows as well. The koi still get their winter (65F and fast) and still get their summer (72 - 75F). Lighting was the biggest concern for me. Even when I had the big pond in Dallas, I still heated in the winter (60F) so the change is not that much different.
Steve