Aloha @Andrew Meikle
Allow me to throw a few ideas out as a resident of the Big Island. You mentioned in your first post an intent to move to the Big Island specifically. Others have mentioned Oahu, but Big Island is pretty unique among the islands and has a number of differences from the other islands. Are you set on Big Island as your destination? Just curious why you chose this island as you commented that you've not spent any time here?
Do you have a place in mind to purchase? Based on your estimate of $300k you are almost certainly looking at Hilo side, unless you would consider a smallish condo in Kona or Waikoloa Village. As many have noted, Hawaii is a tourism-based economy and your juice truck would do much better catering to the tourists. However the vast majority of tourism on this island takes place on the Kona side, which is a minimum hour and a half or so drive depending on where Hilo side you are coming from (if you plan to live in Puna district, like in Keaau or Pahoa area where the cheapest housing prices are, add another hour onto that commute). So my first suggestion as you consider the Big Island is to get more familiar with the geography. There are farmer's markets all over the island, but some cater more towards tourists and wealthy transplants, rather than the working class locals. You will be able to charge a much better premium selling to the first group.
As for others' suggesting the Japanese wedding business, just putting it out there that it would not be nearly as profitable on the Big Island. There are far fewer Japanese tourists on this island than Oahu or Maui. Not to say there are none, but where I can relate to seeing multiple Japanese couples posing for wedding pictures every day at Ala Moana when I lived in Honolulu, here on the big island it is much less common. This is more a general comment as it sounds more like you are looking for a way to make enough to enjoy your passion of juicing rather than searching for other opportunities.
As for your camper van business idea -- I would suggest against it. Similar concepts have been tried by others and failed. While there are many excellent places to camp on the island, there are actually very few proper campgrounds. Most camping is done by setting up tents on secluded beaches or down 4wd tracks. Plus nearly everything is viewable via a day trip from the major resort areas. Those who want more of an outdoorsy camping experience tend to stay at Airbnbs with tiny houses, tree houses, cabins, etc. that cater to that experience. Camper vans would cater to a very small, very niche audience of tourists and I think you would have a very hard time recovering your costs.
As to the juicing business I can't say much about regulations and such since I have no experience in the field. But I do have some close friends who own a small family business selling shave ice, smoothies, fruit bowls, etc. to tourists near Pololu Valley at the north end of the island. They run their business out of a trailer, but permanently parked on their own private land. I can tell you that during busy tourist seasons they have a very hard time getting enough fruit to keep up with their sales. They grow a lot of their own and also purchase from several local farmers and even from Costco but still struggle to find enough when peak season hits. So make sure to take the time to research your supply chain too. Remember, we are on a small island a couple thousand miles away from most everything else. Expect supplies to be limited at times and always more expensive than you're used to now.
One final comment on the real estate side. While rentals properties can be profitable on island, those opportunities are few and far between. House hacking is an excellent option to keep your living costs down -- whether you just have a roommate helping with expenses as you mentioned or if you buy a house with an Ohana unit or even a duplex, tri or quadplex (harder to find on this island because of zoning, but not impossible) and rent out the extra units. But don't expect great cash flow from a pure investment standpoint. You can find cashflowing properties here, but not the kind of cashflow that exists in a lot of cheaper markets on the mainland. In Hawaii, you're mostly having to gamble on appreciation, which is just that -- A GAMBLE. Historically, it's a safe bet long-term, but markets fluctuate a lot so you could find yourself stuck sitting on something waiting for it to appreciate while other opportunities pass you by.
Also right now like nearly everywhere else, the RE market on the Big Island is INSANE. Extremely low inventory, tons of competition especially from mainland buyers (the California exodus is real and a lot of them are coming here with piles of cash to buy a home sight-unseen). Nearly every property hitting the market is getting multiple offers, and most are going for over asking price -- many with cash offers. There's plenty of debate out there about what will happen with the market in the future and I'll let you read the many other posts about that and decide for yourself, but I just want to remind you what you are up against buying right now on the Big Island.
I hope this wasn't too discouraging as I didn't intend it to be. Just wanted to help give you a few more things to think about as you make your plans. Feel free to get in touch when you make it out here. I'd be happy to talk story and I'll certainly buy a cold pressed juice shave ice from you if/when you get set up. Best of luck.
--Paul