I had a similar problem with the septic tank and the leach field.
What we did to solve the leach field issue was to use a European type septic tank system.
The tanks are made of fibreglass, and they have four separate inner tanks, by the time the water comes out into the leach field it is already safe and non-polluting.
Manure is something to take into account because the flies and stench can be an issue in the summer.
I have completed a project not dissimilar to the one you are describing, but back in those days we had no internet to help us out, and we just got on with it.
My projects involved converting a seed store into a house, cowshed to workshop conversion, barn renovation and a lot more.
We decided to go with chickens, as we wanted the better quality manure for growing cabbage for shredding and fermenting.
We even dug out a massive pond to encourage wildlife and have a back-up for watering the cabbage during the hot dry spells.
Getting back to your leach field, make sure that your pipes are surrounded by gravel and wrapped in a cloth type membrane to stop soil and gunge from blocking the drainage holes.
