I don't have any SYS 1 boxes, so I can't confirm they are interchangeable. I do have a Sortimo i-boxx and I can confirm that the Auer assortment bins are interchangeable with those. They are not the same, the profile on the bottom is different, but they do interchange. I do like the Auer containers and would recommend them. I prefer the T-Loc systainers, but for the cost the Auer containers are a better option. My primary complaint is that the Auer containers have more play in the latching and interlocking. When you pick up a stack there will be some slack, a stack of T-Locs feels more rigidly connected. They are more difficult to open and close one handed. You can do it, but the T-Loc is way easier.
The lid on the assortment boxes is also not as rigid. If you shake them roughly and there are empty spots next to the bins, you can displace the bins. The Sortimo containers hold the bins much tighter. I would still buy the Auer containers again because of the price point and the performance is still very good.
I purchased one of each of the tool boxes, plus a 400mm x 300mm assortment box, and a collapsible crate for ~$120. I think it was ~$50 to ship using DHL to the western US. Note that some items are made in different factories, and need to be picked up by your shipper from each location, so shipping may not be as efficient depending on what you purchase.