Re-roofing, roof tiles not possible

There are red Eternit corrugated sheets, Eternit Berliner Welle Profile 5 classic red - 920 mm wide costs about 26.00 Euro per square meter.
Or you tear everything off and put a 24mm wooden battens on and 2 layers of roofing felt. In terms of price, the felt and battens are somewhat more expensive, about 35.00 Euro per square meter.
There are also tile sheets that look like roof tiles.
The tile sheet PS47/1060RT made of steel has a height of 47 mm (35 mm profile height + 12 mm elevation of the next tile), a useful width of 1060 mm, a thickness of 0.50 mm and weighs about 5.30 kg/m². These tile profile sheets are produced to size up to 7.10 m. Price about 20.00 Euro per square meter.
Call a roofer or carpenter, they will make proposals and offers for you.

Ps. there are people who do a roof coating on the old Eternit roof, I don’t think much of that..... old Eternit sheets are brittle and have many small cracks. There is no miracle cure that makes old Eternit sheets or roof tiles tight again and keeps them for years.

A cost estimate is free in most companies ;)

old Eternit panels are brittle and have many small cracks. There is no miracle cure that makes old Eternit panels or roof tiles waterproof again and lasts for years.

Yes, of course it always depends on the overall condition, as well as the family's future plans and usage, among other things. Some possibilities for affordable solutions have already been mentioned here. The problem is that you can get a false impression from material prices alone, because dismantling and reassembly also involve high costs. I would definitely not patch up an overall dilapidated condition, but only if it is at a manageable number of spots that can be properly sealed. Then you don’t need any miracle cures, which is why I also advised against a pure so-called "coating" without a concrete statement on waterproofing. I would only consider it if the roofer or a certified specialist company guarantees the repair of the few spots with a warranty. Apparently, the OP has already had several contacts with roofing companies, as I read. In addition, the parents might not be able to afford a new roof or might shy away from the effort and circumstances and therefore could live with a solution that a typical homeowner would not consider. Therefore, the technical AND personal situation is important in the decision-making process.

Then you could almost sell the house (value approx. 250-280k euros) and build a new bungalow (somewhat smaller, since about 30-40 sqm is lost to hallways here), i.e. with 130-140 sqm, new. Land prices are about 50-70 euros/sqm.

Are your parents saying/thinking that, or is it just in your head? If yes, then seriously think about it... if it really is that "simple" for your parents.

There are red Eternit corrugated sheets, Eternit Berliner Welle Profile 5 classic red - 920 mm wide costs about 26.00 euros per sqm.
Or you tear everything off and put a 24mm wooden sheathing on it and 2 layers of roofing felt. Price-wise, that is somewhat more expensive with felt and sheathing, about 35.00 euros per sqm.
But there are also pan sheets that look like roof tiles.
The pan sheet PS47/1060RT made of steel has a height of 47 mm (35 mm profile height + 12 mm elevation of the next tile), a usable width of 1060 mm, a thickness of 0.50 mm and weighs about 5.30 kg/m². These pan profile sheets are produced to size up to 7.10 m. Price about 20.00 euros per sqm.
Call a roofer or carpenter; they will make proposals and offers for you.

P.S. There are people who apply a roof coating to the old Eternit roof; I don't think much of that... old Eternit sheets are brittle and have many small cracks. There is no miracle cure that makes old Eternit sheets or roof tiles watertight again and lasting for years.

A cost estimate is free at most companies ;)

Well, what can I say. I also found the prices for Berliner Welle and was then surprised when they wanted 33,000 euros for about 260-270 sqm. Of course, including all necessary work (scaffolding (bungalow), demolition work, plumbing work (roof pipe, downpipe, etc.), roofing (including underlay membrane, counter battens, support battens)).
My parents set a price limit of 25,000 euros. Let's see if a roofer can still be found for that.

Say/think your parents or is that just in your head? If yes, then seriously consider that... if it really is that "easy" for the parents.

In principle, that is possible. But they are very selective when choosing the plot of land. Therefore, it won’t be so easy to find a suitable one. For that reason, renovation would naturally be preferred first.

My parents had set a price limit of 25,000 euros.

Only they are very picky when choosing the plot of land.

That can of course be understandable wishes... but whether these can also be reconciled with reality...?

Therefore, it will not be so easy to find a suitable one.

In the end, you just have to be careful that no half measures are created, with which you are then not satisfied either.
Let's see what it will be...