"I like the look of shrubs in front of the trees to cover any gap between the base of the trees and the ground." In many cases, probably most, what are called "trees" will have branches and foliage all the way to the ground, unless it is removed by mechanical means (humans or grazing animals.) You might plan very carefully or hold off on the installation of gap-filling "shrubs" or they may get "eaten" by tree foliage.

You can find seeds for just about anything -- all kinds of plants that are not sold commercially -- on Ebay.

For increasing your chances of success, you may want to avoid exotic plants and go for something that may be common but definitely works. What's growing in your area that would do the job? If you see it, take pictures and get it identified.

Seeds of conifers are not easy to obtain and all you will get are those of the species, the vast majority of which will grow far beyond 15-20 feet tall! And you can extend the "several years" to several decades if you grow from seed as well. It is a long, drawn out process.

Most states have a Department of Natural Resource or similar agency that sells bare root starts or young seedlings of native trees at a very reasonable cost, often for reforestation projects. I'd check into that.

Lawyer Nursery in Montana used to be an excellent wholesale source for inexpensive small tree starts, however they just closed their doors/gates after 60 years due to retirement. I am not aware of other, similar operations, but I'm sure they exist.

I'd try posting on the Trees forum for some additional sources for low cost trees and for better, local suggestions of what to plant. Is water available to all of your 4.5 acres? That is pretty much a requirement for any tree establishment..............

I can't speak to conifers per se, but my experience with trees in general from seed is that they grow much faster than expected, most likely from being never confined into a container or having their roots disturbed. Another advantage is that they don't need supplemental water. They sprout according to nature's rhythm and keep going that way ... provided it's a species adapted to the climate in which it is grown ... another reason for passing up the exotic for the common. The cost is so low that there's little reason not to give it a shot.

I’m in Southern Illinois and we planted 2’ Thuja Green Giant seedlings on two edges of our 5 acre lot. They grow quickly if planting instructions are followed and watered/weeded regularly. We mulched until they were established. They create a natural privacy screen that you cannot see through and the foliage is low to the ground. Ours are 30’ now and planted 9-10 years ago. You should be able to get bulk rates.

These aren’t ours but similar. Ours are fuller. I like the way they look staggered too.