草地 right about time. seed is better than sod for now.
每年都买还是多年生 both
if the garden is small buy the annuelly. It flowers longer, less weeding , looks nice at first year.
多年生 more weeding, flowers shorter, have to wait few years for it to grow.
So you find few 多年生 you like according to shade of sunny. put them apart according to their mature size, fill the gap with annuel.
soil: check the yellowpage, there are many place sell them. They would deliver.