What kind of grass do you have? Bluegrass spreads, fescue is a clump grass, burmuda spreads like crazy. My general answer would be once the wildflowers are up and going they will shade out any grass. Once they are up you can mulch a bit to help them and that will keep out some grass. You may need to watch the grass edge of your bed the 1-2 years. At most you may need to re-cut it once a year.

I have a similar area 4 by 30 with grass lawn on one side and driveway on the other. The lawn grass sends runners into the bed every year, but it is fairly easy to pull the runners out (I have a sandy loam soil there.) Some years I will use a shovel and cut sod, but I am a bit overzealous. The bed becomes more like 5 by 30 or 6 by 30, and then I need new flowers to add to the bed, which then needs to be widened again the next year.

As to annuals, I always start with annuals when I start a bed. It makes it look nice until the cooler weather, when I plant the perennials. If you plan to use seeds, the annuals will help shelter the young wildflowers.

I agree with the previous posters. I have a large circle wildflower area in the center of some lawn. the lawn is bermuda but also just some clover type stuff. I just use an edging type tool like a half moon shovel, and just keep a few inches of trench between the lawn and the flowers. Only certain months of the year when the grass is growing pretty eagerly, is this necessary.

I am not sure what species of grass it might be. In fact, I am not even sure if it is grass or sedges or a combo. I think some clovers are mixed in as well. I plan to use a mix of native perennials from a native plant nursery (not the walmart meadow stuff) and I've collected some perennial prairie flowers to sow in for the future.