ᐅ Converting a Commercial Warehouse: How to Get Started?

Hello everyone, I’ll start with my first post as a new member. We are currently looking for a new home. I am interested in an old commercial or industrial building that can be converted into living space based on our own ideas. About 10 years ago, I did something like this with my parents. The result: for a total price of 170,000 € they completely renovated a single-family house with 150 sqm (1,615 sq ft) of living space and a lovely garden. However, we invested a lot of personal labor. I am now imagining doing something similar. I have already found a property that interests me. It is an approximately 200 sqm (2,153 sq ft), single-story complex with a hip/flat roof from the 1960s, for which the current owner has already applied for a change of use permit to convert it into residential space. I have about 100,000–120,000 € available for renovations. I understand that everything will need to be renewed— all the wiring, windows, possibly the roof, and then the interior fittings. What I am not quite sure about yet is how to best start in order to get a rough overview of costs right from the beginning. Would a building surveyor be the right choice? I want to avoid any unpleasant, costly surprises during the renovation. Oh, and when should an architect get involved? Of course, a few new interior walls will need to be built. Is an architect needed for the interior work, or only when it comes to the exterior modifications? Thanks in advance.