If you mean size of the data then no limits beyond the reasonable ones as the limits of your server or the entire network capacity and its willingness to host your data.
If your server is down and no other node is willing to pin your data then the data will not be available. IPFS by itself does not ensure the data to persist so you would need some external way of incentivizing other nodes (something like FileCoin in theory).
Network participants. It is a decentralized storage so anyone can connect to the network and host data.
Unlikely but in theory anyone who runs a gateway may introduce some kind of filtering on the content stored and served. Obviously it would be counterproductive if accepting and distributing encrypted data.