I'm not a particularly experienced gardener but I've always found buddleia bushes to be easy to grow in any soil. They look lovely and certainly attract butterfly....buddleia is known as 'the butterfly bush'. Enjoy your garden.
Buddleia is a great idea... I would chop it down each year so that it doesn't get too massive. Lavatera is also a favourite. I could also not go without fruit trees as they seem to attract wildlife really well. I am not a big fan of crab apples, so would go for the real thing. Also, a peach/apricot/plum will flower early and help bees to get started.