Congrats to your son on his new home! And isn't he lucky to have your expertise in helping him plan and plant his yard. (and aren't you lucky to have some new area to play with!)

My thought would be to plan to clear everything off levels 2 & 3 first. I'd probably use a machine (brush mower or bladed weed whacker) to cut everything to the ground, cover with thick layers of carboard or multiple layers of newspaper and deep woodchips or other mulch. Then watch carefully and hand pull or use roundup on whatever comes through. In my experience, this won't be much, though you may want to paint stumps with brush killer before mulching. Plan now and replant either this fall or next spring. I wouldn't try to plant while all this mess is still around because even if the viburnums etc survive, it will always look terribly untidy.

If some of the plants on level one are worth keeping, I'd hand weed using clippers and a trowel to cut woodies slighly below ground level and remove herbaceous vegetation and then newpaper and mulch that area as well. Since it sounds like there is a good seed bank of weeds, I'd plant to use corn gluten and remulch annually for a couple of years, especially around new plantings.

Ditto what Babs said she is brilliant.

Only one thing I would add. Considering the sunlight available and the precedent set for the growth of edibles i.e. the raspberries, please consider staying away from chemicals like Round Up. This could be a great location for a combination of edibles and ornamentals and you don't want that crap in your soil. Regardless of what you want to plant, herbicides like Round Up are toxic to the ground water. The world needs less Round Up, not more.

A good rototilling will dig up most of the weeds.

Also,a great organic treatment for stubborn weeds is a blow torch. Honestly, burning them to the ground will snuff them out most times if pulling is not feasible.

Thank you, I'll comment a bid later.

My last home was surrounded with tulip trees, good luck! They'll even turn up in a lawn.

I agree that clearing/cleaning and planning should be done first as nhbabs suggested; this will give him a clean plate and the ability to pull up any volunteers or small tree seedlings while he researches all of your excellent choices for buffer plants.These will that make a strong statement against a forest backdrop. Your son is so lucky to not only have space to work with but also to have your guidance.

Molie