Theres no way to tell you what dosage to take because the potency by weight for opium is extremely varied. It would be a complete guess for someone over the internet to tell you how much opium to smoke.(im assuming your smoking it right?).
As far as the benadryl, yes you can take some before you smoke the opium, and it could very well help with histamine reactions to the opiate alkaloids. I dont know about benadryl helping with nausia, it has never done that for me. Also, it would not potentiate the high of opium)or any other opiate) it would merely sedate you more(if that). And in my opinion, that isnt potentiating your opium high, because its not activating any opiod receptors or anything, its just making you more sleepy. And id rather be awake to enjoy my opium buzz.
Just my opinion.