Welcome and congrats!!! Hawaii is amazing and I'm totally jealous that you are going to get married there!
My personal favorite is the Big Island, bc of the islands diversity...rainforests, beaches, mountains..its amazing. And its not touristy like Oahu or Maui. So that being said, its also not the most accessable (ie, public transportation sucks and its appropriately named bc it is a BIG island!). There really isn't any nightlife on the Big Island and you are limited to beach front resorts. Try looking along the Kohala Coast (North Kona) for the best beach front resorts.
Oahu is nice, def the easiest to get around if you dont want to rent a car and it is prob the island where you will pay the least bc it is super touristy, in particular the Waikiki area in Honolulu. Not my personal favorite bc I think there are much better places to go to see the islands beauty than Waikiki. But its still Hawaii!
Maui is beautiful and I'm pretty sure it is where most ppl have plan their weddings. The beach-front resorts, while fabulous, can be pricey. One thing to note is that all of Hawaii's beaches are public so you might find it difficult to find a resort that lets you have a totally private beach wedding ceremony. Some of the beach front resorts in Maui might be different since they host the majority of the weddings.
When you say accomodations for your guests, I take it you guys are planning on paying for their hotel stays? That might not be done for 5k however, there are a lot of vacation rentals around the islands that you might be able to make it work. There are many places, esp in Kauai that are 5+ bedrooms and even allow you to have your wedding at the rental. Check out VRBO.com and type in weddings..you'll get results from all over the place but a lot of them will be from Hawaii.
Gotta go to work for now, but if you have more questions, feel free to contact me. Good luck with your planning!