They are full size and yet still run off 120v and just need a water source and drain. 

I always buy something I could see myself living in. not necessarily the colours, but the location and size of the home and the condition.

Here are a few examples: https://fourpeaks.box.com/s/heitpydgv3s6gtbiudszgtglhzn7gj29Our park size is usually 40 spaces up to about 230 spaces.

Does size of mortgage paydown become a factor here?

As for why the US, simple - trust, market size, and access to information.

Normally I would say absolutely, looking at the above chart.

Also, with the size of the units, we knew that we could add a bedroom somewhere within the property.

Depending on your practice size and income level, you may just want to start with maxing out 401k and also adopting a defined benefit plan before committing to becoming a lifelong real estate investor.  

Even accurate estimates don't bear out on a small sample size.

On our higher end houses we put stackable in our primary walk in closet I did this in the home I built and live in although mine are stacked they are full size not a combo unit.