We were just there last month and it was the best vacation we ever had! The airfare was the most expensive part, but was still reasonable at $500 from Atlanta. Try calling International Discount Travel at 800-466-7357 and please tell them Gladys referred you. She is the lady who helped me plan my trip and she was wonderful.
There are many affordable places to stay on Ambergris Caye - especially closer to town, and if you don't mind sacrificing a pool. Look at the Sunbreeze, Ramon's, and Rubies. These are all on the beach, but I don't believe Rubies has a pool. It is right in the heart of town, so you may prefer that since you wouldn't have to spend money renting a golf cart to get around.
We found great happy hours at Big Daddy's (5-9pm) and we also enjoyed Fido's courtyard - both are right in town just down from Rubies. We found that dancing and the more "partying" kind of night life didn't get started until late at the dance bars - like around midnight and people partied until 3am.
I think you'll find a good mix of what you're looking for in Ambergris Caye. We went as a group of 3, all in our early 30s and we felt completely at home and comfortable. Don't worry about safety - the locals are very friendly and helpful and I always felt very comfortable, even at night walking down the street alone. Hope you can get good airfare, cause this is a trip well worth taking!