There are a lot of factors to take into consideration . First of all age sex and size of your koi , what are your winter outdoor temperatures ? Then there is the size of your budget . I tried keeping koi in my basment but not a good idea , then I built a lean to greenhouse structure and that was the perfect solution for wintering my fish . Tem was between 44f and 49 for the whole winter and I kept filters running . These fared a lot better than ones in my heated pond as I had to give numerous salt treatments and cost of heating which now is out of my budget as well as constant filter cleaning .
I do not recommend indoor to anyone unless wife is koi kichi , If you do go indoors one has to have light as well as good ventalation or mould can grow . Mould is so dangerous that just a little has closed down portable classrooms in schools . So carefull costderation should be taken as water creates hummidity and one should look at ones health factor .
I did see one exception where there was vapour barriers in a basment ventelation and a wood stove but as soon as the gentelman stopped useing the wood stove he moved the fish .
Regards
Eugene