HelloI am about to buy some plants for my cottage border

Hi, heuchera don't like heavy soils, so try to improve it with lots of organic materials, such as well rotted manure, garden compost, spent mushroom compost, or get some soil improver from the gc, you could also add some horticultural grit, to improve drainage, the soil ph, should be neutral to alkaline, Derek.

I grow Heuchera on clay, improved as derek advises and they do very well but I find they prefer some shade, especially the green and yellow leaved ones.

Start off with the red ones and see how you go.
You could plant paler ones in containers and move them into a shadier spot if they look as if they are suffering.