Hello,
What did Muhammad (PBUH) own?
you need to read his biography.
yes, he had a roof over his head. owning a house was easier in those times. houses were simple structures and did not consist of ikea furnitures, tables, chairs, tvs, computers, coat hangers, dvd players, light bulbs etc
What did he do with Kadija's wealth (sorry if I misspelled her name) or the wealth from his trades,
at times he freed slaves, at times he fed the poor, at times he reinvested the money into the capital earned. He was a self-sufficient businessman after his marriage to Khadija before prophethood, but he did not lead an extravagant life.
and from conquering other civilizations etc..?
that came way later on, that money was used for the betterment of the state or given to muslims as war booty which was a common practice in those times because there were no salaries for soldiers.
Where did his children sleep and/ or in what conditions? I heard he had a camel/ horse (not sure which one) and a sword - was there more?
you need to read his biography
He ruled over Medina, right?
i've never heard of this one. all important matters of Medina were voted upon (Mushawara in arabic, roughly translated as "taking suggestions or advice"). Muhammad was not the king of Medina. His job as a prophet was to give the law of Allah in the exact form as revealed to him by Gabriel. matters concerning the state were frequently discussed in Masjid-e-Nabawi in congregations.
How did he handle the treasury of Medina or any other provenience within his control?
the baitul maal (state treasury) consisted of zakat tax, personal contributions from himself and the people of medina, taxes and investment income. this baitul maal was not the property of Muhammed (pbuh), it was the property of the state. it was used for the betterment of the state and to provide food and clothing for the poor.
people in those times were not as wealthy as the middle class man of todays times. the number of rich upper class individuals was very little and the gap between rich and poor was very high. people's purchasing power was very little in those times and there were not many items to purchase such as playstations, televisions or cars.
building a house was easier since houses were simple and did not consist of chandeliers or light bulbs.
it was a simpler time