In my opinion, you can forget about these steel boxes. I looked at one at a customer of the provider and was not very impressed. The customer wasn't either when I asked him. Sure, the price is great, but basically it's a bunch of scrap metal that you just place somewhere. I would first take a look at the garage and then think further. As far as I know, 5.85 is the exterior dimension. Will both your cars still fit in there? A concrete garage has to be calculated with at least €15,000, and if you want electric gates and electrical wiring, maybe a door and windows, you’re looking at nearly €20,000. The rest is handled by the foundation contractor.
You don’t need to declare your €20,000 equity to the bank... that’s the money you will need for hidden extra costs. So to speak, a small financial buffer.