Is it permissible to go to Hajj using funds from a company that previously engaged in Riba (usury) but has since stopped, or is it obligatory to go using one's own private funds, and is it necessary to inform people of one's intention to go to Hajj?

There is no doubt that your Hajj from your own private funds will bring you a greater reward, due to the Prophet's (peace be upon him) saying: "But it (the reward) is according to your expenditure or your effort." If you perform Hajj using company funds, your Hajj is valid. Company money is mixed, lawful and unlawful, and its use is permissible. You are not legally required to inform people about your travel for Hajj, and it is recommended to bid farewell to family and friends.