Nicolas, It is hard to believe that in 5 pages of posts that no one has asked and you have not offered what the budget is that won't allow acoustic treatment. I'll ask, "What is the budget?".
Here is my take. If the budget is $10,000.00, put half of it into treatment. A $5000 system in a treated room will sound 1000% better than $10,000 system in your nightmare room. Adding treatment is labor intensive, and perhaps is a reason that you say there is no money for it.
Forget about the speaker system, for now. Put 50 people, 100 people, 200 people, etc in that room for a sit down dinner and the sound will be deafening just from the conversations. Now add the speakers with music for an event. The noise floor will be incredible. One that your budget probably can't overcome, in terms of just shear volume required. You are in a complex predicament, and your reputation is on the line with this client. Make sure they understand, fully, the limitations of their request.
