The first thing I look at to position a coop or run is drainage. You are right, a wet area is asking for problems. If water drains to it or stands in it, you will almost certainly have issues with it stinking, plus a wet coop or run is an unhealthy area. If the area drains well your life will be easier.
I've never kept cats penned like that and I don't have guineas so i can't comment from experience on that set-up. I suggest you really think about your convenience, which I think you are. As long as it is dry you can build something they are happy with so location doesn't really matter to them. You have received some good comments. The things I'd consider are where will you store your feed to carry to them, either the whole bag when you get new feed or daily when you fee them. Same thought process with water. Will you be wading through water if it is raining, so not just does the coop/run get wet but do you getting to them. Think rain as well as snow. They can be noisy. That might bother some people, others it will not. Do you want them seen from the road? That might make a difference in some neighborhoods.