If you go with dexters, buy them from a calm herd (go visit the herd, walk through the field etc), otherwise they are bloody stubborn and very flighty and will jump fences. I used to dislike Dexters greatly, however after sitting for little acorn on here, they arent all that bad, some are nice.
Cattle with horns, are just as dangerous as those without horns.. just have to give yourself more space when handling them. Some horned cattle i've worked with in the past didnt realise they had horns, and would try and gently nudge you in an affectionate manner.. and you'd get poked by horns (but gently), and others knew they had horns so would nudge you with their nose. They were Welsh blacks, lovely beasties, good covering of meat and tasty.
If you can do a deal with your local farmers go with their breeds for now, that way if you get into trouble you can ask for help.