I have Fayoumi's but I'm in Florida, so heat is more a concern for me than cold. Mine do tolerate the hot, humid summers better than many of my other breeds. I found this thread (old thread), someone knew someone that had them in Maine and said they were fine...
I think silkies, polish and seabrights are known, or widely discussed, as being more susceptible. There really is a long list. I only have one Brahma left, lost all the others. She is the one that every one of her chicks has become symptomatic. I lost all but one wyandotte, others have said...
Just sharing my experience, and yes, Marek's sucks. If you don't have confirmation, get it. As stated, other things can mimic Marek's symptoms, it's better to know for sure, than to guess. Marek's birds are more susceptible to all kinds of other illness due to their weakened immune systems...