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low maintenance
If you want to contain the size and ensure a good crop you will need to do at least biannual pruning (ideally annual) to keep the shape and cut out the intersecting branches. An easy way to think of it is that you're trying to make a basket shape to let the light in and removing any that cross each other.
In terms of a space efficient apple tree there are fruiting ones called ballerina that are elegant and look lovey with blossom in good years.
I'd like to plant some trees in the garden.
Got space for 2 or 3 apple tree size trees.
Ideally low maintenance, fruit or but bearing and good for wildlife.
Should I just buy some apple trees as big as I can afford or is there something better? And where to buy - online, or a decent garden centre?