Renovation of an old building from the 1950s

Hello!
My uncle and my mother inherited the house of my late grandmother.
Key data of the house:
- Semi-detached house built in 1955 with two 64 sqm apartments and a converted attic.
- Gas heating (radiators) from around 1985 (Vaillant VKS 11 and VKS 17) WITHOUT central hot water supply.
- Windows from the late 1970s.
- Gas boilers in the bathrooms are also from the 1970s.
- Electrical wiring and pipes from the 1950s.
- Roof redone in the 1980s, but not insulated, except for a layer of glass wool in the attic.

First, the water/sewage pipes and the bathrooms are to be renewed.
My uncle wants to renovate as cheaply as possible because he wants to rent out his share.
For this, he only wants to renew the cold water pipes and sewage pipes and realize the hot water supply for the bathrooms using electric instantaneous water heaters.
Does that even make sense, or would it not be better to replace the heating system at the same time and opt for a central hot water supply?

Best regards

Christian

Thank you for the answer! Meanwhile, a heating engineer was there. And he actually recommends instantaneous water heaters, as apparently laying the hot water pipes would be too complicated. And the heaters are still "good" since the exhaust gas values are still correct. He apparently does not want to sell anything. I have recommended to my uncle to ask a second heating engineer. Let's see if he has the same opinion. Christian

Just suggest to him that he contributes to the annual additional electricity costs caused by his great idea ... after all, the so-called "Energiewende" is by no means completed!