I would start with the bare patches and sow as you go.
I've been doing this with a 2.5 acre patch myself. Sowed the first seeds a couple of years ago but a really dry spring meant we lost a lot of seeds - also, we're not at the land to scare pesky pigeons off. Whilst a lot of the annuals failed, the perennials took and last year it was full of oxeye daisies, plantain, rattle.
There was NO way I could clear and sow 2.5 acres, so we just began in strips and have now sown about 10m wide in a strip going across the centre of the field. Because it's low on grass, you don't have to scythe back in the summer and can allow the wildflowers to set seed for longer before an autumn sowing instead. That allows natural spread.
Good luck! It's just beautiful when it starts to grow and it's amazing how fast it turns from looking a bit disappointing to being filled with colour!