Not everything can be installed wherever you want. Downpipes and the slope of the pipes already place some limits.
You should also be aware that a potential buyer might see the DIY work more as a depreciation than an appreciation. After all, they don’t know anything about your skills, and even if they did, liability coverage would be difficult.
Thinking further. Does such an apartment block even attract tenants who want a clean and modernly renovated apartment and are willing to pay in full but are satisfied with an apartment block?
Keyword location, that’s always what counts.
I don’t want to reinforce a cliché, but a house in a working-class neighborhood that targets well-off people will probably also be difficult to sell at its actual value.
Some don’t want to live there and others don’t want to pay the value :)
Generally, I would say a renovation that goes beneath the plaster layer is not worth it.
New paint, new tiles, etc. will constitute most of it. I would only do anything else if absolutely necessary.
A new owner might see it the same way: why renew and pay for something if the old would have lasted another 10 years or more?