how do i model a building inside and out!!! help!

hi all!
i’m makeing a short film for my 11 year old sister for her birthday (in november). i’ve got some characters and i’m struggleing to make the buildings and background. for example, one scene is in a castle and the film shows both inside and out. how should i make it? should it be 2 seperate models or 1 big one? and if one big one, how do i join the inside and out? then what about the sceary outside (sorry if i have spelt that wrong). all together, it would end up being huge and i only have 4gb (my pc died and i’m useing another computer so i can’t save anything on it and i’m useing a thumbdrive).