I have read that asparagus is difficult to start from seed. That has not been my experience! It's the easiest think I've grow. I start them in a small flowerbed near my house. I can sell them the next spring for about a dollar each (probably could get more, they sell fast). This past year I kept them and replanted our very-very old aspragus. bed that was weed-choked and starting to lose plants.
I'm not sure that a raised bed is helpful or beneficial for asparagus. They root deeply. The county extension info I've seen says to dig a trench to plant them in. I think the soil is prepared like 2 feet deep? I don't remember exactly because I didn't follow the instructions. . Your county extension will likely have information on how to grow various foods and crops in your area. However you prepare a bed for planting will depend on your soil and climate.
I planted my year-old roots 18" apart. I plant seeds about 3" apart.
The deeper you dig that trench and prepare the soil, the longer the bed will last. Properly prepared, asparagus beds can last for 20 years or more.
The reason raised beds are often recommended is that it helps with drainage; asparagus does not like wet feet. If you have a light sandy soil, don't bother with raised beds, but if you have a heavy clay soil you might want to do the raised beds.
We grew some from seeds a few years ago and they didn't do well, but reading this info here, I know why. The soil we planted them in was not great and we didn't prepare the bed deep either. Lesson learned!
Well, I'm with @Chic Rustler here! Not a big asparagus fan, but give me a freshly dug tater any time! They are soooo good when they are just out of the ground, especially the small ones. Though that said, asparagus is definitely worth the effort it takes to get a good bed for them.