I live in Maui and my fiance and I are getting married here in December so I have done extensive research into wedding/ceremony spaces. As far as finding a large house on the beach, it may be a little difficult. But, check out vrbo.com. I would recommend the south side of Maui because it's absolutely gorgeous and sometimes a little less windy than the west side.
Also, as far as getting married on the beach, there are many beaches that you can choose from. You just have to apply for the appropriate permit and make sure they allow the number of people that are coming to your wedding at the wedding site. Also, the only thing with doing it on a public beach is that you are not able to reserve the beach or any section (so you may have random people in the background of photos and strangers watching the whole wedding take place.)
A backyard bbq is a great idea. However, you have to find out the "quiet hours" as far as a DJ/music goes. And this may be a little ahead of your planning, but we are using Cafe O Lei to cater our reception. Not only is their food delicious, but they also have a variety of island food and are decently priced for buffet style dining. If you need any other help/advice just let me know! Best of luck with planning