With a building like this I would never, ever do anything without an expert, we're simply too inexperienced for that.
Exactly the right approach. You can also assess energy efficiency quite well with some research and experience – building fabric from the war period (actually nobody really built houses then) + unclear annex should be thoroughly examined.
and have already shown the full range of renovation possibilities. In principle, the house looks habitable, no damage, everything works, it gets warm, plaster and paint on the walls, etc. So, currently you absolutely do NOT have to renovate anything, but many things suggest themselves before moving in. You can also gut the whole house down to the shell and install the latest standard and some luxury, and if you want, sink €150,000 or more. It’s all a question of requirements and circumstances. I would tend towards a balanced middle ground.
Regarding the energy certificate, I wouldn’t worry:
Windows most likely from 1980-1990 (you can easily check – it’s always stamped or printed on the spacer bar running around the glass in the sashes – that is the silver rim around the glass). For good frames, just replace the glass, otherwise the whole window.
Glass replacement costs €100-130/m² glass, so about €4,000.
Complete replacement: €400-500/m², so €20,000.
Exterior wall probably 24 cm double brick row + plaster, plus radiator niches, with 11.5cm wall thickness. This will be the biggest weak point where a lot of effort is needed to improve it (ETICS).
With thicker walls (just measure), it looks different. The wall structure of the annex or extension is questionable. Maybe there are documents available?
ETICS: €150/m² wall, about €20,000 – probably the most expensive item, but with ETICS you can leave the radiator niches as they are.
Otherwise, definitely close the radiator niches; for that, you'll need to install new radiators and pipes.
Costs: own work + construction adhesive + aerated concrete.
The roof is definitely still quite young and hopefully properly insulated, so no need for action (very major cost factor).
The heating is definitely written off, especially if no central hot water generation is available. As a single person no problem, but with wife and child it will be expensive in the long run. If you want to retrofit hot water, you should install a new heating system at the same time, maybe you have to anyway. For hot water you will need new pipes, so you might as well replace all others at the same time.
Costs: heating €8,000, radiators €3,000-5,000, pipes (water, heating) €3,000-6,000.
We already covered bathrooms: it will be around €15,000 somewhere if you have everything done without luxury, especially tiling and waterproofing cost time and money.
You need to look at the electrics. Maybe nothing needs to be renewed, or just a few extra sockets, or complete replacement.
So anything between €0 and €10,000 is possible.
Without electrics, you end up with about €50,000 - 60,000 including ETICS and new panes. Another €10,000 for small stuff and reserve.
Unclear:
Foundation / statics, especially with annex?
Basement? Available? Dry?