for kit brews the 20l urn isnt needed, u could do extract brews or partial mash brews like you suggest diluting with sterile water in the FV.
To bew a fully ag brew tho u would need to boil down in batches as your going to start with a diluted wort from the mash and sparge (soak n rinse), getting a concentrated sugar solution from grain isnt as easy as not diluting a tin of extract as much as you want to in the final pot.
all the FV's are fine for use, u dont Need an airlock, just a lid to cover..
get a few kit brews going as all u will need is a few basics (thermometer, hydrometer, syphon...) and bottles to fill :) if you enjoy the hour or so kit brewing and cleaning and sanitising,, then there is a good chance u cld enjoy a 6-8 hour brewday