If you mentioned the climate your son and DIL are in, I missed it, but I would recommend two YouTube channels and a book: The book is Grow a Little Fruit Tree, by Ann Ralph. She tells how to make even a standard fruit tree stay the size you want it, with preference for short enough to pick without needing a ladder. The YouTube channels are David the Good (who is in southern Alabama but whose advice and growing principles can be applied anywhere), and Huw Richards, who is in Wales, but again, most of what he does can be done anywhere other than the desert.
As far as how many fruit trees and berry bushes, I would say, plant as many as you can fit on the property. With some nut trees (hazelnuts and chestnuts can be kept small like fruit trees; possibly others can, too). Then do the vegetable garden underneath the trees and bushes. Check for harvest dates. If I'm looking for apples, for example, I want to choose a really early variety, a later one, and a really late one - at a minimum. That should give you apples for several months, and if one or both of those two late ones stores well, you might be able to have apples through the winter. You can do that to some extent with almost everything. Some blueberries ripen in June (depending on your climate), and some not until September, and others fill in the gap. And so on. Some types of fruit don't have a long season - mulberries mostly ripen in the summer, not into the fall. And persimmons mostly ripen in the fall and even into early winter. So you might only want one or two varieties of those (unless, like here in Kentucky, mulberries and persimmons are the fruits most likely to thrive).
Speaking of bushes, someone up there was talking about raspberry bushes? Curious about that, because all of the raspberries I've seen grow on canes. Is there some other kind?
Anyway.
Also, I would suggest planning in some animals from the start, even if they don't intend to have any right away. Rabbits and quail take up almost no space at all. Chickens and ducks don't need much, and can be quite beneficial *if managed correctly.* All are extremely useful for turning weeds, prunings, damaged fruit, and so on, and turning it into high-quality food for humans. If they do end up on a suburban lot, there will probably be some restrictions on what they can have, but it shouldn't be too hard to find out what those are. Also, given a choice, buy in an area where the neighbors already have poultry, or rabbits, or whatever the son and DIL think they may want to have later on. Less potential headaches that way.
Another thing to plan in advance is a garden shed. Perhaps a greenhouse. Access to water. Fencing. Paths. A play area for children, if they have any or plan to have some. Sketch the whole thing out on some large graph paper, cut a few templates the size of various trees and berry plants, and use the templates to figure out where things should go and how many will fit. (Keeping in mind that the trees can be pruned to any size you want.)
Once they have a plan (which can, and probably will, change over time, but it really helps to start with a plan), they can buy the plants and put them in as they have the money and time. Don't have to do it all at once.
