I have worked FOH Management in banqueting, functions and events for most of my career.
In my experience, to offer everything ala carte with varying prices is asking for trouble. It would be like a restaurant manager sitting 150 guests all at once.
You should have packages that offer say, set entree/set main or set main/set dessert. Then you can add a three course variant of set entree/set main/set dessert with additional cost.
Next, you have an add-in charge for any optional choices.
Ex. $55 for set two courses. Add $6 per person for choice in entree, $9 for choice in main etc.
The other option is to offer an alternate drop menu at no extra charge.
(Around the table: beef/chicken/beef/chicken and so on). This is annoying for floor staff, because they have to explain that guests swap meals amongst themselves if they didn't get the meal they would prefer. You'd be surprised how many people become allergic to chicken in these circumstances.
Buffets should be priced at three tiers also. We used a "hot fork" buffet, basically stews, pasta bake, curries, soups and salad with bread and perhaps two dessert options on our lowest tier.
Next was more things like chicken breasts, roast beef with gravy, seafood chowder, veggie bake, wider selection of sides and more pastries and desserts.
Then was a "supreme buffet" with prawns and oysters, roast beef, lamb and pork from the spit, pates and gallettes, artisan breads, more exotic salads and sides and a veritable feast of sweets including a chocolate fountain etc.
I'm not a cook, but some tips for SOP's.
Prep, prep, prep. Everything should be timed accurately and ready to go when called away.
If you are doing large numbers and the menu is set (or even with choices), organise a crew line.
So, your dish hand stacks the amount of warm plates you need. He passes them to the next guy who plates the mash potato, then on to the next guy who places the vegetables, then the next guy who places the protein component, and lastly to the head banquet cook who sauces the plate and perhaps wipes (although as expediter, I used to do this myself)
FOH needs to ensure a steady stream of wait staff coming through to pick up and deliver. We used to get about 500 meals out in about 12 minutes, on a good day.
But having everything prepped and ready to go when needed is essential.
Having a production line system means that any hold up crashes the flow almost immediately. Have any dietary requirements prepared before hand,many seperate from the line. One FOH member handles all the dietaries and doesn't have to bother the production line looking for the wheatgrass special.
Most importantly - NO CHATTING ON THE LINE!! Eyes down, bum up! Communication should be between expediter and head chef only.
As far as working with outsourced suppliers such as DJs or theming companies, you should create preferred supplier relationships that you can offer the client. Many clients walk in to organise their function with little more than a budget. Having suppliers that you can recommend and you trust helps them a lot. Try to get some kind of cream off the top from them. So, for example, a DJ quotes you $500 for an event, you charge the client $600. Make sure that any and all outside contractors adhere to your OHSW policies and are aware of acceptable load in/load out times.
Your % food costs should be at least 10% less than a full all carte restaurant set up. There is good money in banqueting. You should have little waste, and a smart operator will know how to re-use any leftover food (put leftovers on the restaurant specials board the next day, etc)