I've been researching the "cater your own wedding" option and this is the best guide that I've found so far and probably the most realistic way to cater for a 150+ guests. The visionary and author, Andrea, has put together some very specific details including check-lists on her blog Forkable.
Her approach was to put together a fresh gourmet sandwich/salad bar filled with items that could be prepared in advance and served cold. She ended up with a great spread for less than $1500 which is a great deal! She does give a series of warnings including 1-people will tell you that you're crazy, 2-you will need a lot of volunteers, especially on the day of the wedding, 2-it takes meticulous planning and list-making to pull it off.
But here it is. It can be done!
Photos from Forkable