If you have never tried this before or are unfamiliar with growing fruit trees, it might be simplest to just start with one that is already trained as an espalier. These should be available at better local nurseries in late winter or early spring, which is also the recommended planting time. Growers of these trees have already selected for appropriate rootstock as well as compatible varieties both for cross pollination purposes as well as regional suitability.......not all apples grow equally well in all areas. That is also why I suggest looking locally - started espaliers are difficult to ship due to an awkward shape and size and local suppliers generally will offer multigraft varieties that will work best in your location.

If the demand is not great where you are, these may be a special order plant. But late summer/early fall is when retail nurseries place their orders for the coming season so speak with your nursery buyer now to make sure they can bring in what you want.

The only time I've ever seen espaliers in garden centers are when they're abbreviated, skinny versions on a diminutive trellis ... all of which are designed for shipping concerns. I've never seen anything like what the OP has in mind for this project. IMO, the big fun of an espalier, is creating something outstanding from basically a little whip of nothing. The most difficult part would be creating the trellis to attach it to. The rest is just fussing with tying and some ongoing regular pruning. If one could find a started espalier in a garden center, it's going to be quite expensive.

They are routinely sold as I described in my area......but then this is a major fruit growing state. No trellising - just an already trained espalier, typically 5-6' from tip tp tip, ready to plant and train against your own trellising system. And usually a 2 or 4-way graft.

Fruit trees in general require a degree of knowledge beyond basic gardening skills to grow successfully. If not with these skills and experience already under one's belt, then one is faced with combining this need for knowledge together with learning how to train and prune seedlings or whips into an espaliered form. That's a lot of info and techniques to put together all at once and expect success. If already an accomplished fruit tree grower, then the espaliering may not be as big a learning curve.

Thanks for your thoughts, @gardengal48 ! I'm definitely interested in purchasing an espalier that's already been started for me. But wanted to have the lowest branch about 28" off the ground, so it's not blocked by whatever I put in front of it. I understand I'm unlikely to find a pre-trained espalier with bottom branches that high. But let me know if you know something I don't! I will also talk to local nurseries and see what they say. Good tip.

Totally understand about the degree of knowledge required here. Ideally, I will be able to find or order something that's already been started. If not, I'm willing to hope for the best and be ready to try something else if I need to. ; )

Thank you @Yardvaark ! I do not have another space that is espalier-fruit-tree-friendly, but I will take your design thoughts in mind. I could do a simpler, symmetrical shape between the small window and the big one, and just have a smaller tree overall.

You said the upper branches will be the fruit of scion- but if there are 4 varieties grafted on one trunk, there are 4 scions, right? How do I know which varieties will appear in upper branches? (It's not crucial, really; I'm more just curious!) Thanks so much!

24-30" above the graft/root flare is not an uncommon branch placement for a ready-to-go espalier, with the next layer a similar spacing above.....and so on. Different wholesale growers will have different methods or practices but you should be able to find - or request - something that works for you.

"How do I know which varieties will appear in upper branches?" It would be highly unlikely that any commercially produced espaliers are are arranged as a graft onto a graft onto a graft; but much more likely that they are all grafted onto a rootstock, whereas everything above each graft is its distinct fruit. The former (unlikely) scheme would require a year or so to finish the grafting. In the latter (likely) scheme, all the grafting would be done within the same day.

Wow, thank you for the detail!