Xesthir,
You missed out on one of the biggest factor, what is your budget?
I think 750mm is small enough, don't make it smaller base. Even if you get a 600mm wide fridge, i think you won't really notice the gap of 75mm at each side. How often we look down and see this? I think my base is 800mm and my fridge is 680mm wide.
In the end, you can do without a base. It's not really necessary to have one. On the contrary, having a base is quite a trouble if you want to clean the floor of the fridge. As nowadays fridge had 2 front rollers, you can just move it out quite easily, but with a base it is not that easy.