First house purchase, year of construction 1962, how much will a renovation cost?

Measures for saving energy are also mentioned in it.

If he or the seller takes the cheapest one from the online offer, it is about as meaningful as the roll of paper next to my toilet.

But there is no energy certificate. I know it is mandatory, but I am not sure if the seller will not lose interest if I want it to be created at his expense

No matter who feels like what: An energy certificate is mandatory and the notary will demand that it is available.

Regarding the prices: Just ask your family/friends what the work costs each, then you have a framework for it. Water pipes/sewage pipes are a piece of cake in terms of material; it is rather the labor that costs.
Here in the "house building" forum, mostly new builders are active and there self-work usually boils down to crossing out and building the garden shed.

We renovated our 140 m² place with 60k (heating + pipes + radiators, underfloor heating in kitchen and bathroom, electrics, triple-glazed windows with warm edge, flooring, screed, attic insulation, basement ceiling, basement exterior waterproofing, interior doors, bathroom, all water/sewage pipes, masonry work for openings and windows, final plaster on all walls, interior paint)
Year of construction 1959, 17.5 cm stone, 2cm EPS, 11 cm klinker: gas consumption before 60,000 kWh (without hot water, due to instantaneous water heater), after 22,000 kWh including hot water (about 3-4,000)

We subcontracted heating, water pipes and windows. That also cost a fair amount.

Do not underestimate the time aspect: I was working full-time and a new dad and my wife was at home. During the renovation, my father was always with me and we worked every weekend for 5 months full 2 days (7-8 hours each), on Fridays always procured materials or took care of small tasks and I also had about 4 weeks vacation that were used only for this. Afterwards, I was done and vacation ready, my daughter half a year older and luckily most was finished. But it could not have been more! And I only had 39 hours per week and 3 minutes from home to work or to the construction site. Ideally, you do that during 3 months of parental leave in summer. We had winter without central heating during the renovation.

We also completely renovated our house built in 1976, or rather, we are in the process of doing so. Some things were done by companies, others by ourselves, simply because we didn't get any or any reasonable offers. For example, companies wanted 20,000 euros just for insulating the facade on the upper floor! We then bought the materials for about 5,000 euros and did it ourselves (fleece-coated mineral wool boards on battens, counter battens, and a wooden cladding). Especially the wooden cladding was hell ... I had bought untreated tongue-and-groove profiles, it was supposed to look like rhombus cladding in the end (so with a gap). That means the tongue had to be painted black – this took 20 minutes per 4-meter batten – and then glazed on both sides, twice on the front. I spent 3 days on 5 battens. Almost 200 were processed and the east side is still missing, meaning this is incredibly time-consuming. But not particularly expensive. For the insulation and battens I spent 1,000 euros and for the wooden cladding 2,000 euros, stainless steel screws were another item (several hundred euros). And of course the scaffolding per side was up for a good 9-10 months because I did everything alone. So another 1,200 for the scaffolding (only south and west, north is at ground level). It is possible to save massively if you only need the material, but it costs time. If you have helping friends and family unlike us, I think it doesn’t matter so much.