After shtf a big sailboat would be a tremendous BOL if (big if) there were no pirates and looters. A large sailboat is like an ultimate RV that can give you world wide mobility.
The obvious place to start is with a lesson. I would go to the body of water I intend to use in the future and look for an instructor or school. Start on a boat about 22′, the handling and comfort will be closer to a useable overnighter. The body of water you will be on will dictate what type of boat can be used to a large dgree. Sheltered water vs open water, shallow bays & lakes or deep water.
Most smaller sailboats are sloop riged. Sloops are the easiest to master since they only have two sails. You really don’t need to worry about rigs because a sloop Is probably all that would be available until you charter a big boat.
Start some place where there are no strong currents and big tides to contend with.