I started my own catering company a little over 2 years ago on my own (had catered with others before). I just had my 2nd biggest wedding yesterday that went from disaster to success. I'm still recovering and learned many new lessons. This is kinna long but I wish I knew these things starting out.
3 is the magic number.
Figure three appitizers per person, per item. You have 6 menu items 3 x 6=18. 18 bites of food. That is a healthy amount. If you had 10 items I would go down to 2 each. Most people want a good sized plate full of food and that's it. Some take more than others. Some don't eat much. And some don't take any of a particular item, so that kind of works out on it's own. Use that as a basic guide but go with your instincts. So I would figure on 450 meatballs, shrimp, etc.
3 is Still the magic number
Estimate the cost for all of your ingredients, do a test run through the grocery store or delivery catalog if you have to. Take that number and times by 3. I don't know the cost of ingredients in your area, but a good guess would be around $500 in food and supply costs (disposables, etc.). I would quote them $1500. Thats $10./person which is very reasonable and you should make enough for your time. You are being a mobile restaurant. It's hard to get a good meal under $10 and you should be paid for your time.
You WILL need helpers. Maybe not cooking, but delivery and set up are a lot of work and if it's mobile, food cabinets and bags are very heavy with food. For 150 guests, I would ask two friends or family members to help you out that day.
Be Confident: I still have trouble with this sometimes.
It's okay if you run out of an item. Just make sure all the guests have had at least one pass through the buffet. If people want seconds, and you are out just say "I guess that was good" or something similar to build your reputation and not that you ran out. I've lost menu items (lost all my eggrolls in the parking lot loading a delivery once). Just explain what happened to that dish. And be honest. You can't control the whole world, things happen. If they are a little upset give them the cost of that item back.
This is the most important step
Prep everything before if you can. Have the equipment and supplies you need boxed up a few days before the event. Do your food prep 2 days before your event. The day before cook everything. All you want to worry about the day of the event is reheating, delivery and putting final touches on everything. Fresh everything is very hard to do even at the venue kitchen. You want hot, hot. And cold, cold.
I hope this helped you a little. I wish you luck and have confidence you will do great.