Make the basic structure first (walls, floors, ceilings, windows and doors). Once you've given up on that, you'll give up on all the models and prefabs you mentioned.
look I made my school map. download it from my thread. look at OK I MADE A REALLY BIG MAP, I WANT MY CUSTOM SKINS FOR HOSTAGES. or named something like that. i put a link for download. you need about a month. and a ton ov nerves.