Usually, yes, they can be stacked with a sold-separately stacking kit, but it depends on the model. If you have one of those front-loaders where the controls are on the top-back, similar to where they'd be on a top-loader, not so much. Primary things to look for when creating a stackable combo (outside of getting good machines to begin with) is that the washer and dryer are compatible to stack (getting a matching set usually resolves this), and that whoever is operating them can still successfully get to the controls / get all the laundry in/out. This is a bit of an issue for me, as I am a shorter person. Outside of that, just make sure that the stacked height fits where you want it, and that you've got hoses & vents long enough to reach the hookups.

As a general note, a washer and dryer with built-in stabilizing technology (like these have) help with the whole stacking thing, as they won't vibrate anywhere near as bad.

The "wash quality", from what I've noticed, doesn't really change based on whether or not you've got hot or cold water in the drum -- I usually just use hot water for sanitizing things. However, if you've got something in particular need of help (grass stains, mud, etc), put it on "Heavy Duty" with cold water. It'll take a while to run, but it'll take out nearly anything. (See my general review on this item for more details)

From what I understand, the internal water heater is primarily for the steam features, but knowing Samsung, it'll probably kick in with some supplemental oomph if your plumbing's water heater isn't spewing hot enough.

My unit came with a big "10 year warranty" sticker for the drive unit on it, so I'm going to say yes.