My partner and I have a similar problem with the difference of 50k equity and 5000€ net income. The problem is that even with a shell construction, the costs are not fixed, or do you also have a lawyer or building expert who reviews the contract? Or if you are currently single, who guarantees that a partner would want to move into the house? Maybe they work in the south of Berlin and the house is in the north? I also see it critically to build a shell house and then have a construction site for several years. How do you want to hold your friends accountable for craft defects? That can strongly strain a friendship or your friends may have families, or in a few years, interests change quickly. Better save some money first or live, live and live and travel, see the world.