Doing 5 gallon batches I originally went with a 5 gallon cooler mash tun. It mostly works, but there were enough times that it didn't. Because of that, I went to a 10 gallon cooler.
Wish I would have just started with the 10 gallon.
I decided that controlling temps was important to me. I think it makes a difference for me but I guess it all depends on each person and their climate and/ambient temps.
I'm using a Danby fridge and temp controller.
I've used my tap(well water) up to this point. I have a 7 stage 150GPD RO/DI system for my reef tank so I'm going to run a line before it hits the DI and start using RO and minerals. I figured since I already have the system, might as well.