I am a similar size.5 11 and 210. I flew a Model C Speed Wing for a year while I was rebuilding another plane. It was tight for headroom and did not allow a helmet to be worn. There are some photos of a raised roof on one of the other forums. When I rebuilt my Model IV I had been for a ride with Dessert Fox4 and stole all his ideas for my four. I did the wide body mod, the Az baggage compartment, and speedster tail. The difference in comfort between the Avid and Kitfox is night and day. I think the seatpan makes a huge difference but I also find way more headroom. You will fit in the Mark IV but your comfort may suffer. While I prefer the Kitfox seatpan there are some that think the Avid sling seat is the way to go.
If the Avid is a great deal and in your price point go for it, you can modify it and tailer it to your comfort level. You will fit but you'll have to work at getting it comfortable.
Oh and now that I think of it what is your comfort requirements. Do you drive a high end vehicle with all the bells and whistles or a small three cyl economy vehicle. What is comfortable for some may not be comfortable for me and vice versa.
Good luck with your choice.