The basement is not a good option unless u have a lift, or unless you have a double storey house that has driveway access to the bottom area. (I dislike double storey houses anyway).

The huge huge house with an internal gamesroom is the BEST option, but building costs need to be looked at.

The seperate shed is fine,make it a big bastard and u cant go wrong. More room the better. Allow for up to 30 machines cause you are over half way there Pete from the photos I saw of your collection last !    By the time u finish collecting, you will use all the room for sure !