Wildflower and Grass advice

Hi,

I'm soon to be helping the local council plant up a communal area of land. We have put in an orchard already but I'm a bit out of my depth when it comes to anything beyond edibles so I'd really appreciate advice from anyone with experience of planting grasses and wildflowers.

The planting will be along the south side of a long ditch (not sure how long but a few hundred metres at least) which will capture and drain rainwater from the site, on a mound raised 1m high then sloping 6m down to the ditch.

An attractive mixture of resilient/self-seeding native flowers and grasses that will attract wildlife and not cost a fortune to plant would be ideal.

Grateful as always for any suggestions.

Cheers, Andy