If you can get a conversation with them, which may have to be by paying for a ‘pre-application’ visit, you should find you get lots of useful input and guidance on the sort of thing your planners look favourably upon, and which not. Don’t expect them to approve every application for things they say they like, but it gives you a good insight into how they view things.
If your ultimate goal is something they would like to support (especially if it creates jobs and or brings money into the area), they are likely to be happy to work with you to help you achieve it.
Would that that was true!
I'm sure there will be some planning officers who are helpful and will tell you what kind of enterprise they would possibly support. But in my (admittedly limited) experience of three different authorities, the default answer is "You can't just live on a few acres of land. We would not support any sort of planning application, so it's a waste of time trying." Their job is to inspect planning applications and decide "yes" or "no". Not to provide you with helpful suggestions on how to successfully navigate the planning jungle.
I believe the right to live in a caravan while you are building a barn is correct. In fact, I believe it applies to any building that you are putting up and it is for security reasons. Obviously, with an agricultural venture, a barn is the only thing you are likely to initially get planning permission for, and that is why it is commonly mentioned in that context.
As you say, 5 years is a long time for the building, but the permission is actually phrased as only valid for "up to" 5 years, so people don't string it out for ever.
Without wishing to sound bleak - agriculturally related businesses are not the same as agriculture and do not require the same necessity to live at your business premises. Like you say, it is possible to make a living with a few acres. Traditional starter farms are pigs, poultry, calf rearing etc where the animals need a 24 hour presence. I know someone who makes a living from hatching and raising pheasants, and he got planning for a log cabin on site.