We got the rubber mats from our mb dealer - fair price and very good quality.
Hook up lead - I think it’s best to buy a 25 metre lead. Although quite often you won’t need the full length, sometimes you do, especially abroad. We already had the 25m lead and bought a 10m lead, which is actually all I’ve needed so far.
Some people have a 10m and a 15m lead with a connector, so they can use any combination they like! I find it’s awful to have choices!
Mattress topper - I would wait until getting the van before deciding if I needed it if you are only using the top bed. We’ve found it very comfortable without a topper
One thing we’ve found very useful is the Comfortz external window screen cover. It keeps the screen free from condensation and helps to keep the van cool, as well as making it dark. The one provided with the van looks like a bit of an afterthought although it does provide privacy and if the front seats are not swivelled round is easy enough to fit. It won’t help stop condensation though.