A Wikipedia article confirms that the park of the Paris Foreign Missions Society is the largest private garden in Paris. With regard to the acquisition of the property, the article in the link below says this:
The Seminary (Séminaire des Missions Étrangères) was created in March 1663, when Mgr. Jean Duval, ordained under the name Bernard de Sainte Thérèse and nominated Bishop of Babylon (modern Iraq) in 1638, offered the deserted buildings of his own Seminary for Missions to Persia, which he had created in 1644 at 128 Rue du Bac.
The article gives detailed and fascinating information about the history of the Seminary, part of which says:
At the end of the 18th century, the French Revolution halted the growth of the society, which had previously been very rapid. At that time it had six bishops, a score of missionaries, assisted by 135 native priests; in the various missions there were nine seminaries with 250 students, and 300,000 Christians. - Paris Foreign Missions Society
With regard to funding, the Society had the backing of the Church and the pope as well as the King of France. Other income appears to have come from charitable donations. As to how those missionaries were able to endure lengthy and often hostile postings, I can only conclude that they were made of sterner stuff than folks today. Well educated and self-sufficient, they were no doubt able to organise the local communities and enlist help to provide for their daily needs.
When Jesus sent out his disciples, he told them not to take any gold, silver or copper with them; not even a bag for the journey, nor an extra tunic, sandals or staff. He simply said “the worker is worth his keep.” They would find accommodation from worthy persons in the towns or villages they went to (Matthew 10:8-11).
The missionaries sent out from the Paris Foreign Missions Society may have left France with very little in the way of material possessions, but they had the backing of the Church and the State.
Here is a useful article on the history: - Society of Foreign Missions of Paris (Catholic Encyclopedia)