There's no way I'd be able to have a tank and not put fish in it lol, even if I had the space for it.
Out of the fish I plan to keep, the betta is the largest at around 7cm, and even then most of that is fins. Since I'm not keeping massive fish, shouldn't need a massive tank. If any of my fish do get ill/pregnant, I'll panic about it then. Probably buy one of those cheap plastic tanks and try and get hold of a cheap second hand filter/heater when it happens.
I know it's better to get it together sooner rather than later, but right now I'm paying out massive amounts of money for stuff, and my tank is still only sand and gravel and water. It's an expensive nothing at the moment.