You can get more for your money when the bareroot and bulbs come in. You want to get them soon after they arrive as stores don't always store them the way they should. You can order bulbs from catalogs which cost more, but you will get a guarantee of quality bulbs and they will ship it to you at the right time for planting, you just have to make sure your bed is ready to go.
Dahlias from seed will bloom the first year, tubers will over winter and can be divided
Daylilies will produce fans on the leaves and clumps should be divided every 2-3 years
Chrisanthemums can be grown from cuttings and they make good cut flowers
Some annuals can be used now to fill in. They will last most of the summer and some of them can reseed and come back year after year.
You can get some good plants if you know where to look. Ask friends if you can get cuttings or divisions from plants they have that you like. Sometimes you can get plants at garage sales, but you may have to inspect them for well as they may not be in the best condition. Never get a plant in the dark and always isolate new plants for a month to make sure they are healthy and bug free. This includes plants from nurseries.
You don't have to fill the entire space at once. In fact, it is better that you prepare the ground first to give them the best chance of surviving.