Things to consider:
Size of your tree will be determined by the rootstock, variety of apple is determined by what is grafted on to it.
A seed from a Liberty apple will not produce a totally new apple, it doesn't matter where in the orchard the tree was located. When creating new varieties, apple breeders pick two parent varieties and then raise thousands of seedlings in huge greenhouses and narrow the selection down over 10+ years or so and don't always come up with a winner.
Are you going to make cider? Want some apples for pies? Just want some to eat?
Does your area have late frosts? Then you probably don't want an early blooming variety.
Descriptions of apple varieties usually include bloom time and these will also overlap each other.
Not all varieties can pollinate all other varieties, look on the description for recommended pollinators.
Also some varieties can withstand frost with less bud damage than others.
Some varieties (like Liberty) need less spraying, but that doesn't mean they are a NO SPRAY variety, you'll still need some kind of spray program. The Japanese beetles were really bad in my area this year, for example.
Semi-dwarf trees are usually planted about 12' apart. Dwarf trees can be planted about 2-4 feet apart.
You'll spend more money putting in dwarf trees per acre, but you'll get more fruit and the spraying/pruning and picking will be easier.
It sounds like the "fruiting wall" system would work for your yard, but you need plenty of sunlight.
Each dwarf tree will need a stake or you'll have to build a trellis. Don't skip this part, high winds can break young fruit trees. If you have deer in the area you'll need to provide some protection from them.
You also need good drainage, I learned my lesson in soggy, heavy clay soil so don't plant trees in a wet spot unless you can find a way to drain it.
I've gotten decent trees from Stark's, get on their email list, they run sales all the time, and you can get trees in the fall and plant them before winter.
I also recommend Cummings nursery in upstate New York, their on line catalog has lots of great information.
There may be local growers in your area. I have a very small Amish fruit tree nursery in my area that has a thriving business with no internet sales, he doesn't even have a phone, so look around, businesses like that are out there.
Apple trees from big box stores probably won't be dwarf varieties, so skip them unless you have lots of space.
Check out you tube videos for all kinds of tips on growing/pruning/spraying apples.
If you want to make cider, there are many excellent books that have sections on recommended varieties, but remember that the most important thing is to choose something that will grow well in your area.
$4 a pound is an ridiculous price, but you are paying for the orchard experience, so that's why people do it.
I can get top quality apples around here for $20/bushel, which comes out to about $.40/lb. Seconds are $7 a bushel or less depending how much you buy.
The reason you want to grow your own is that commercial apples are always picked before they are fully ripe to prevent damage when they are handled and shipped. Tree ripened fruit has way better flavor and you can really tell the difference when you make hard cider.
Use google maps to find other orchards in your area or even make a fall road trip a little farther away. See what the professionals are growing and if you talk to the farmers and show some interest, they'll tell you all kinds of stuff you'll never learn from any book.
