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 !  

This is the spot that causes the brain to short out  
I already have 27 games, and I think I will stop at around 40  
if I allow 3 square meters per game, that means I will need 120 square meters of space
that is not counting space for some chairs, maybe a small bar, toilet (so visitor need not have to go into the main house)

Maybe I should change and become a CCC, and just get the top 10 games in Minty condition