I know European ash tends to be slightly better than American white ash. But good European ash is capable of 120# I'd say. Maybe a bit more even. American ash can undoubtedly show large variations in density as well. So your best piece of American white ash might very well reach European ash. All I'm trying to say: ash can show large variations in density and strength.
Ash is excellent in tension, but not so great in compression. I'd go for a flattish belly and a crowned (or trapped) back. Monitor set closely as you tiller. When set dramatically increases, it's likely you're pushing the drawweight too much. Ash takes excessive amounts of set, rather than breaking. So when set kicks in by the time you're thinking "but I'm still miles away from my drawlength or -weight!" then you need to lower your drawweight. I'd rather have a 90# low set bow, than a 120# bow with 4" of set.