I am coming in late here, but it seems there are still quite a few unanswered questions about the protocols, so I thought I would just mention a few things. ( I was a network engineer and had worked with both Z wave and zigbee before ever purchasing smartthings for use in my own home.)
First, the forum already has multiple FAQs on Wi-Fi and zigbee co-existence. I would suggest reading those. You can find them in the FAQ section:
Next, while zigbee uses the same frequency everywhere in the world, it allows for a number of different “profiles” and these are not all interchangeable. They don’t even use the same addressing scheme. Smartthings uses the “zigbee home automation 1.2” profile.
Beyond that, different regions put different legal restrictions on the transmission strength that zigbee devices are allowed to use. For example, the US allows a signal strength of almost twice that allowed in the EU or the UK. Consequently, some zigbee devices which are legal to operate in the US are not legal to import or operate in the UK.
Zigbee is used a lot in Europe, but not generally for low-cost DIY systems because of the challenges of deployment so that you avoid Wi-Fi interference, particularly given that boosted zigbee cannot be used, Just generally restricts the size of the market. This is discussed in the zigbee versus Z wave FAQ in the community – created wiki, as it’s also probably the reason why Z wave is so much more popular for fixed location devices like light switches in low-cost DIY systems in the US.
http://thingsthataresmart.wiki/index.php?title=Z-wave_versus_Zigbee
If you only want to use Zwave devices, don’t select SmartThings as your hub. You will be much happier in the long run if you choose one of the hubs which specializes in Z wave. This is even more true if you intend to have an extensive network of more than 200 devices. Just as one example, SmartThings provides no network mapping utilities at all to customers. None. But the most popular Z wave – only platforms in Europe all provide some kind of network mapping utility. And most handle multiple controllers well, which SmartThings does not, which allows you to expand the number of devices.
Vesternet is a good place to research the various zwave platform options.
I had a few other thoughts on some of the specific points raised in the conversation above, but I don’t think they would necessarily impact your own project, so I’ll skip those for now. 
p.s. Specifically with regard to zigbee ZHA 1.2 devices for low-cost Home automation systems in Europe, there are a few options which are not carried by Vesternet which are quite popular with SmartThings customers in the UK.
These include the SmartThings brand of devices, Orvibo, and Hive. Osram also has some, although they include some special features which will only work with their own Gateway.
Schneider has a really excellent line, but they tend to be more expensive and only available through professional installers, so I don’t know many smartthings customers who have used them unless they already had that system.
Xiaomi is another option from a big Chinese brand, but they are only intended to work with their own Gateway and have a lot of what network engineers would call “idiosyncrasies” when used with a smartthings hub. They are still popular because they are so inexpensive, But they do tend to work best when deployed in a small installation, under 30 devices all within 40 feet of the hub. So more for an apartment than what you’ve been describing. People who try to get them to work through repeaters and in larger installations tend to run into a lot more problems with this brand. But you can research them in the forum if you are interested.