The standard Duna Multy is often a good cage for a special needs dwarf hamster. I think a set up for Snowy needs to be thought out carefully, which I am sure you will, to avoid too many changes needed later. Would it not be possible to just get another Maxi Duna Multy for Pau and adjust Snowy's existing cage. She might enjoy the space if it's easier to get around and there is just one deeper bedding area - so she can opt between easier movement and burrowing as she feels like it. I know the Maxi Duna is a lot more expensive. Or there's the Zoozone large for about £80, but the lid needs meshing.
I'm just wondering if she might do better in the same familiar cage but with one end easier to manage. In a way it would be better not to have levels or shelves at all with her mobility issues, but have more space at floor level. As she seems to use the end near the wheel more, maybe the other half could be more level and less bedding for easier walking about? I went with the hemp mats on top of substrate so the height was still there, but a level surface to walk on.
It's difficult to weigh up the stresses - a cage move is stressful and a cage rearrangement is stressful. But if only one end of a familiar cage is rearranged, it could be less stressful as she's already familiar with the cage and has one end the same as before.
She may be quite speedy on the flat! As she is when running in her wheel. And it could help her mobility. Sorry to put a damper on the idea of a cage with a shelf but I'm not sure levels would be good - she could fall going up a ramp - and while she may not hurt herself - it would be stressful if she struggles to do things.
