Repave driveway, drainage, costs?

Honestly, I can think of 100 things, if not several hundred, that I would do first.

The problem itself is not urgent, but very likely the avoidance of its avoidance is: namely that the resident lets someone come who takes care of it at shameless prices. It would therefore be appropriate to interview her about what problem could even exist here: "when driving into and out of the garage, there is rattling near the threshold" or "perceived balance problems when walking across the driveway" (both of which Bolek from Thomas Freitag's "Polen am Bau" fixes faster than any all-service from the phone book).

Remove everything, including the substructure (about 30-50cm), put in recycling, compact it, add gravel, and new paving on top.

Would there be any objection to reusing the paving?

The problem is more the ups and downs than the appearance - although, who knows how it will look once it's freshly mixed...

Honestly, I would know 100 things, if not several 100, that I would do first.

What would those be?

Since the downpipe is going to 'disappear' anyway, you're already working on the driveway anyway :)

Would there be any objection to reusing the paving stones?

No, you can easily reuse them.

What would those be?

A lot of things that are important to YOU.
The paving stones don’t look so great with the waves. So what? They serve their purpose.
If you have the money and the appearance bothers you, then go ahead and have it done. Otherwise, use your money for more important things.

Oh, I would know quite a bit, but unfortunately I cannot/may not do as I would like...

that is around 8.5k EUR gross, or 6.7 "cash" :(

You are aware of the consequences of commissioning such, right?

Are you aware of the consequences of commissioning this?

sure. You can already tell from the following smiley.

Just because of this offer, the company is no longer really in the running, so far there are just no others :(