I don't necessarily think a nice hotel Cantonese restaurant is too far out from the OP's budget. Assuming 20 dishes for 20 people, there are many nice hotel restaurants where you could keep the average price of a dish to ~250HKD (eg, $150-200 for rice or noodle dish, $300~350 for meat/prawn dish). Loong Yuen at Holiday Inn would be around that price range, and it's a very decent place.

Another interesting option is something like Rainbow Seafood on Lamma Island. I think their set dinner menu was something like $600 for two. Would be a great experience to ride a boat out for a seaside dinner in a small island village.