Until you actually apply for Planning Permission you can't know if you will get it. Pre-guessing the result is surely non-productive. The place sounds ideal for you and as you have animals there already it would be a shame to get rid of them before you know if that is going to be necessary.
Getting permissions and then building are some of the most stressful parts of life - it never goes smoothly and all you can do is keep looking towards once it's all finished.
Don't give up without a fight
Also, even if they turn you down the 1st time you can always re-apply.
Sometimes it is worth looking from all angles, I know someone who applied for permission 20 years ago, was refused but still lives there
See if you can get a copy of 'field to farm' as it explains all the different ways you can do exactly what you are wanting to.
I have had the bad side of the enforcement officers, I had to rip down my static but I'm still here and fighting
Is your landlord in support? How about the farmer who would be able to see your new dwelling? If you can get support (especially written) from him/her it would be fab, the councils don't like hassle. If you can show them there won't be any grief from neighbors that will be a giant tick towards them giving PP.
It shouldn't be like that perhaps, but it is