I'm renovating a 4000+ victorian home in the midwest. It hasn't been lived in for over 9 years and currently has a gravity furnace, which no longer works. I have no idea which type of furnace/AC unit I should look for or what I should look for. Any suggestions would be helpful. Thank you!

When you say gravity furnace, do you mean hot water or steam boiler? If so, you could just pipe in a new boiler, but then you have to worry about ac. Without being there to see the layout of house, its impossible to recommend best options. Good luck and please let us know what you decide.

Always get at least 2 estimates and better yet 3. See attached file to save yourself some trouble.

Make sure that each contractor does a load calc (aka Manual J) to make certain they sizing the system properly. Be sure to discuss your planned renovations that will make the house more energy efficient so they can take that into account when sizing the system. It would not be unusual for someone to have a properly sized system before renovation and inadequately over sized afterwards. Oversizing costs extra during initial installation, cost more to operate and can adversely affect longevity.