Nightmare: Weed Garden

I have a big hill in my backyard that's about 25'x20' and it used to be covered with a honeysuckle and briar jungle. In retrospect, I should have let well enough alone. At least it smelled good.
When I put an addition on five years ago I had the contactor tear out the jungle in expectation of completing the plan I had from a landscape architect. When the estimate for all the plantings and labor came in at $16,000 (!!!) I knew that would never happen. But ever since then it's been a constant battle with the weeds. (I did manage to plant veggies up there last year and learned I love Buttercup Squash even more than Butternut.)

But look at the nightmare now!!! Apparently dandelion seeds and the giant mystery weed love a cold snowy winter! Please feel free to suggest super-dense, low care, fast-growing, full-sun plants I can get so the weeds have nowhere to land. I'm thinking mellow yellow spirea, day lilies, rhododendrons, some forsythia and pretty native flowers that are already growing in several spots elsewhere. I also plan to get a few arborvitae to plant in a row to block the view from the house above. Arborvitae grow about a foot a year!

I hope to someday have a garden as beautiful as my friends Anne and Scott...