You might also consider Asko. I don't know what the measurements of the Miele that you are considering are. We just installed an Asko W6222 washer and T712 dryer. They are 24" machines, so not full sized. They are stackable, but we installed them side be side. The only issue with these machines so far is that I had to reverse the dryer door ( which is possible on this dryer) and am still working through some issues with that). Otherwise, we are very happy with the Askos. If you haven't used front-loaders, be aware that the cycle times are longer, the detergent required is different, you use much less of it, etc. Yes you will need an overflow pan. Whether it needs to drain somewhere may be a matter of local code; your plumber should advise you on this. This is an installation issue, as the installer has to get the washer (or both machines) up over the lip of the pan. Hopefully you are getting professional installation, so they can deal with it. Differences in dryers are that some are vented and some are condensation. The Asko T712 comes in both versions; ours is vented. Do that if at all possible. If you absolutely can't vent the dryer, please search and read other posts here on condensation dryers. They don't sound too good to me.

The Miele 4000 series laundry appliance are not stackable. Miele does, however, make several smaller washer and dryer pairs that are stackable, or that will fit under a standard height countertop. Check the Miele website for details. They refer to them as standard or large capacity machines.

Why aren't they stackable? I mean... the controls are in the front -- what is to stop me from finding a similar sized dryer and putting it on top and affixing the two somehow?

The 4000 series are not stackable because Miele didn't design them to be stackable. In order for laundry sets to stack, they need to be specifically designed so that the dryer interlocks with the washer below. In the case of Miele models that are stackable pairs, this requires removing the top of the washer and the installation of a stacking kit, allowing the two appliances to be securely fastened together. I'm guessing Miele didn't design the 4000 series to be stacked because the stack would be too tall for most people to see or reach the dryer controls at the top.

Stacking unmatched appliances not designed to interlock in a stack would potentially be dangerous, because short of welding the two units together, there'd be the danger that the dryer could fall or slip off from the top of the washer. Washers and dryers shake and vibrate while in operation, increasing the risk.

There is, however a way to accomplish a variation on a stack. It would involve building a very strong shelf above the washer on which the dryer would rest. The shelf would have to be substantial and be tied into the framing of the walls or supported with strong timber or steel posts. You would also need some method of securing the appliance so that it couldn't slip off, if vibration caused movement. Going this route, the dryer would be above the washer, but technically, they wouldn't be stacked.