Old thread, but I'll chime in for anyone else considering this. I haven't converted a pool myself, but I've done a ton of research on building ponds, and converting a pool is absolutely doable.
The depth is actually a big advantage, especially in cold climates like Vermont. Koi need at least 3-4 feet of water to overwinter safely, and your deep end gives them that. The shallow end is great for plants and for fish to warm up in spring.
The main things you'll need:
Aeration - a small pond aerator or fountain keeps oxygen levels up and prevents stagnation.
Filtration - a bog filter or a simple external pond filter helps handle fish waste and keeps the water clear.
Plants - lilies, cattails, and marginals help absorb nutrients and keep algae in check.
Gradual filling - don't rush the process. Let the water sit and plants establish for a few weeks before adding fish.
I got my koi from
Next Day Koi a few years back, and the fish were healthy. I keep ordering more to expand my gang fish lol. Once the pond is cycled and stable, they'd be a great addition. It's a rewarding project if you take it slow.