Where are you? You might get better ideas from folks near your area.
I embarked on a similar project about 12 years ago. I made all the mistakes one can make, then learned from all that and went forward. One mistake was to use a 'wildflower mix' and assume that was going to be satisfactory. It wasn't.
You'd be better off getting native seeds from folks in your area and start germinating them now. In fact now is maybe the ideal time to start. Put a posting out on the Seed Trade forum.
You might want to bring in a load of good dirt for the area. If however, there is quack grass in that plot, you will be ripping everything out later on and starting over. But that's just the learning process at work.
From now on it will be fun, fun, fun.

Thanks for the tips. I'm in Twin Cities and my seeds are mostly from Prairie Moon. I'm not using mixes, but putting together various seeds so I have control over what is going in there. As I mentioned above, I have actually started germinating seedlings (well almost, I'm still stratifying them) and will be planting those in addition to some direct sowing. My main concern is with soil prep. We're having some really warm temps coming up and I'd like to start the prep (ie tearing up the landscaping cloth) and do some sowing while it's still someone cold to get the natural cold stratification that most of the seeds need. My big worry is soil quality and weed suppression. I think the garden was put in at least 5 years ago so many of the weed seeds present may no longer viable.

Ok on all. Nice to know your in the same area as I am, almost. It was my experience that I could get at least ten times the seedlings by germinating indoors, then planting out the seedlings-in a protected environment for one year. Just planting them in the ground can be a waste of one growing season.
--- The weeds will always win! ---
But you need to find that out for yourself first.

One good thing about natives is that don't need much soil prep. They should be able to grow in the soil that you already have.

The other trick is to keep the bed mowed for the first growing season or two. The mowing encourages the natives to keep busy setting roots and the weeds will be getting their heads chopped off, so they cannot produce more seed. when you stop mowing the next growing season, the natives should have enough root growth to out-compete the weeds...

Good luck with that.

You mentioned that the garden has landscape cloth right now. It is possible that the cloth may have killed off the weeds that were growing there prior to it being placed. You may have some seeds in the soil still that could germinate, but it shouldn't be that big of an issue. I'd just go ahead and plant a quality seed mix if you're looking to go cheap and have limited time. Any weeds that pop up shouldn't be a problem to pull in the first growing season with it being such a small area.

If you do want to invest in quality plants that will establish quickly and give you blooms quicker you can buy plants that are already growing in pots. If you space them correctly you should be able to do this for a little over $100. It can be a major time saver and you will be able to identify which plant is which right off the bat. Many native plants are simple to divide too and you can fill in any spots that look bare the following year with the plants you've selected.

Also, planting seed mixes that contain grasses during the Spring will lead to the area being dominated by grasses. If you are set on planting seeds, make sure that you don't a ton of grass seed in the mix. If you were to plant it in Fall it wouldn't be that much of an issue. If I were you I would leave all grass seed out and only use nursery grown or wild-collected grasses. This will allow you to control where and how much grass develops in the area. Big Bluestem and Indiangrass look great in small areas if there are only a few plants. Too much of them and in 4-5 years it will be all that youll have growing in there unless you use control measures on a regular basis.

Thanks for the tips all. The weather is so nice right now I went ahead and pulled the landscaping cloth up. The soil underneath looks to be in pretty good condition to I went ahead and mixed up my own native mix from various seeds that I have. I marked off several areas where I put only grasses (Indian, little blue stem and river oat) so I'm not mixing in grasses with the flower seeds. I'll be starting the seedlings indoors in about two weeks so I should be able to transplant those come late may and I'm thinking that combing the seedlings and the direct sown stuff I should be able to get a nice mix.

I'll keep things short this year to promote growth. This area is small enough that I can mess up. There is a large area of lawn next to out woods (full of great natives!) that we'd love to convert to native prairie growth so this should be a good learning experience.

For it to look more natural, you might want grasses mixed in with the perennials. Let it come up, then see what you think.