I also find all of this way too vague. An uncertainty of 2.5k€ in income makes a huge difference for such a project. What would the existing properties possibly cost? It’s much easier to discuss that within the family than with the neighbors. It’s also not a great topic: "Hey, if your mother is no longer here, we would like to buy her house from you, what would you want for it then?"
Does it mean that for the grandparents' house, your wife’s parents have a 25% share because there are four children? Or would your wife have to share the 50% that her parents inherit with a sibling? The more people involved, the more difficult it naturally becomes. Especially if other heirs/grandchildren are interested in taking over the house, it can lead to a nasty conflict in the family.
Otherwise: with your 5k net income you can think of a loan of about 500k€, possibly more if your wife is expected to earn as well. However, if indeed four children are planned in total, it will become very challenging again – and realistically, your wife will barely contribute to the family income over a very long period. Beyond that, the goal now is to save as much as possible – in three years, a nice sum can still accumulate. The good thing is that presumably no real estate agent needs to be involved in any case, which saves a lot of additional purchase costs.
And as concrete figures as possible are needed: how much money must be put down initially to buy the existing properties or to pay out the other heirs? Renovation/conversion costs come on top.