Rough cost estimate BJ55 renovation

First of all, regarding the purchase price – nothing special for a good location. But it looks quite rural with comparable surrounding buildings. But you probably know better how the prices are over there.

100 sqm living space is probably 70/30 in ratio ground floor to upper floor, so a significantly larger footprint. Also has a basement.

Roof insulation: If you have a few hours, do it yourself and save 12-15,000 euros.
Roof covering: why?
External insulation: For that price, you can insulate the house to new build standard.
Internal insulation: Who does that and why?
Basement ceiling insulation: Do it yourself for half the price.
Heating incl. radiators: What kind of heating should be installed? Take an air-to-water heat pump if you’re doing everything anyway and tear out the screed/lay underfloor heating. There’s not much price difference. Here too, you can remove the screed in EL.
Window replacement: Should be enough for all windows (plastic) and front door.
Electrical work: Probably will be more expensive for solid work (there are plenty of electricians on specialized portals – of course with an invoice).
Doors and frames: Should only cost half for really good doors.
Bathrooms: That can be way too much or way too little...
Floor coverings: fine – but please don’t take plastic!
Kitchen: Depends on your wishes.
Other: Depends on your wishes.

My tip: Invest in an energy consultant and head towards a KfW efficiency house 85 or better. You’re already doing so much by doing that.

To keep it short, I come close to my estimate with a bill of about €150,000?! (So purely related to all the work and not the house itself)

Should one also bring in an architect who takes care of the trades or rather go it alone?

I would manage with that – as I wrote in my breakdown. But I also do a few things myself to have a buffer.

Do you have any experience with the matter? Renovation is not rocket science. However, you have to take care of it intensively if you want to handle it yourself. An architect or expert takes that off your hands.
The architect is actually only necessary if you want to intervene in the substance of the house (move walls, add an extension, dormers, etc.). You need someone to find craftsmen for you and supervise the work. Or you do it yourself... (the architect usually works with "his" craftsmen/companies – but you can also look for individual craftsmen on e...Kleinanzeigen or MyHammer – but of course, you have to be careful and not be guided by "being cheap is cool").
Although you can’t really have an electrician/plumber supervised by the architect.