That sounds like a brilliantly improved cage, and yeah rotastak only works when you want to practice diy too lol
There is no minimum bar spacing, but if shes a bar chewer then I'd avoid the really tiny spacing.
I just went through a few ebay searches and can't find anything with full levels, unless you want a chinchilla cage with wire mesh shelves and those'll be really bad news with a hamster unless you keep them covered unfortunately. So right now my advice is to skip ebay lol
Oh and just for the record - bin cages can be surprisingly cheap... If you're willing to take hours of work on them anyway.
Ours was made in the space of about two days with the only equipment being two penknives, a poundland screwdriver and a permanent marker. That said, I wouldn't neccessarily reccomend that building process to anyone lol. It was very er... exhausting to be honest, so if you can find and afford a big cage then going for that instead'll be brilliant for Mona.
Oh and I love your forum name
We're fully aware that we're Scamp's humans lol, but we've convinced her to share us as long as they're only getting attention when she's asleep or out of her cage lol. Unfortunately she does routinely wake up to tell us off if someone else is getting attention and she notices, so she's not quite got the rules sorted yet...