How to make cross-border purchase?

You could use a parcel forwarding service, but I'm not sure how well it would suit your fiscal needs..? I mean, if you're buying from dell.de, both countries are in the EU, so I'm not sure what difference it would make.

If you're using a forwarding service, billing address on the invoice will be your own, so you can effectively deduct the expense if that's what you mean. If you buy from outside the EU I know you can have back VAT and all, but there will be some import duties to be paid, so I'm not sure if it's still worth it?

You cannot -- and bear in mind that purchasing any system not sold in your home market will result in the need to return the system to the country of purchase before service is rendered.