The single level is good as long as it's big enough
I wouldn't get anything smaller than the 75cm by 45cm type cage linked above. Because you want to fit a lot of "enrichment" into it. Enrichment isn't just toys it is textures etc. So for example when our Syrian was older and everything on one level he had a hyacinth tunnel (they can chew on those as well as walk through them) A coconut hut (they love sitting in those and grooming because they're round). A large cork tunnel - those work really well even for small dwarf hamsters because it fills up the cage and gives them overhead cover a nice big tunnel to run through and they like scratching the inside which is good for keeping their nails short - and it's also something to climb over that's not too high. Cork is an interesting texture for them.
For nesting, plain white toilet paper is best - a few sheets torn into strips. A little heap in the cage somewhere but not inside the house - they like to forage for it and may pouch some to take back and refurbish their nest. Keep topping up the pile. It's also safe if chewed or swallowed. It gives them places to go and things to do and favourite places to sit and lots of variety.
I think of their cage as their habitat.