1] Buy fruit trees on miniature rootstocks.
The rootstock determines how big the tree will grow to & is the most important aspect of your decision on what to plant. These days most fruits can be grown on very small restrictive stocks which make them suitable even for small gardens.
You must check what you are buying and be sure that they are on a rootstock appropriate to your needs. Some non-specialist outlets may not label the stock their trees are on, and this could be a vigorous rootstock that is far too big for your garden.
Trees on miniature rootstocks yield early in life and are easy to manage. Apples on M27, Pears on Pyrodwarf or Quince C, Plums on Pixy and Cherries on Gisela 5 are all ideal even for small spaces & can be planted just 6’ apart. This type of tree does prefer a good soil so make sure you incorporate plenty of compost or leafmould and make sure they do not get too dry during the summer.