What should I consider when building something myself?

Hello dear house building forum members!!!! :p

A little background about us.
In December 2010, we signed a contract with a small construction company for a semi-detached house. The plot and the house were included together in this contract. The other half had not yet been sold.
Construction started in April 2011. The construction company wanted to build the unsold semi-detached house at the same time and sell it either during the construction phase or afterwards.
In August 2012, the company filed for bankruptcy.
Now almost everything is done and the property will finally be transferred to us. A notary appointment is scheduled. The other side is now being sold to acquaintances of ours.

My future neighbor works in a construction company himself and gave me the idea to complete the house with our own labor. At least the shell, electrical work, tiling, and landscaping.
Many of our relatives have also offered their help, and the craftsmanship is there, as three houses have been built by relatives themselves 10-15 years ago.

So we wanted to give it a try.
But what do I need to know/arrange beforehand?
What about the helping people? In case of an accident? Is there such a thing as accident insurance and how expensive is it?
How do I prove in a possible inspection that the helping people are only friends and relatives and not illegal workers? Do I have to register them somewhere beforehand and how expensive is that?

Is there anything else important I should consider?

Hello,

So we wanted to give it a try. But what do I need to know/settle beforehand? What about the helping persons? In case of an accident? Is there such a thing as accident insurance and how expensive is that? How do I prove during a possible inspection that the helping persons are only friends and relatives and not illegal workers? Do I have to register them somewhere beforehand and how expensive is that? Is there anything else important I have to consider?

Nothing personal, but these are questions that have already been answered here several times and there simply isn't time to repeatedly start over. Read through some of the posts and your above questions will answer themselves ;)

Kind regards