Bathroom in 80s style - Redesign - Costs?

Hello everyone,
the biggest and presumably most expensive modernization will be our 11sqm bathroom. The plan is to keep the structure (i.e. the length of the sink, tub, etc. as it is). Next week the plumber is coming for a cost estimate: I want to be a bit prepared for that.

Questions to the group, do the pipes need to be replaced? How can I, as a layman, tell if they need to be replaced?
How should I imagine it, will all pipes be replaced then, meaning the drainpipes and the ones in the walls?
What can such work cost or be allowed to cost?

For cost reasons, is it possible to simply tile over the old tiles, or does it make sense to gut the entire bathroom?

Thanks to all!

Gutting and refurbishing would be my approach. I do not know today's costs. Our bathroom of the same size cost around €15,000 all in about 9 years ago.

Gutting and redoing would be my approach.
I don't know today's costs. Our bathroom of the same size cost about €15,000 all in 9 years ago.

So I can calculate costs for tiles, furniture, toilet, bathtub, washbasin. I can’t assess the plumbing "services"; I’d prefer not to do more than necessary and pay extra.

We have now received an offer. The plumber has always quoted a cheap series from a brand (probably house brand). Quality is good, design is not acceptable at all (in our opinion). The prices at the bathroom showrooms are not justified, so we are looking for series / manufacturers that have good quality and are not completely overpriced.

Do you have any ideas?

The plumber has now always priced a cheap series of a brand (usually the house brand).

Then take a look at the other series of the house brand online. And if it just doesn't work taste-wise, it just gets more expensive.

Then take a look at the other series of the house brand online. And if the taste really doesn't work, it just gets more expensive

What do you mean by that? I don't always find out which manufacturer has a private label, do I? Do you possibly have any recommendations?