Beginner needs help!

Hello,

as a true novice to planting trees, I would really appreciate some advice. I would like to plant either a cherry tree or an apple tree (for my son's first birthday) in my mother's garden in Dorset. However, she has lost two trees to canker, one quite recently. I am determined to make sure we take the right steps to ensure we look after this next tree properly and do what we can to help it survive. Any advice on the following would be truly appreciated:
1. Apple tree or Cherry tree? Which is the hardiest? and what species would be good for eating apples or cherries?
2. Site for planting (how far away from the lost trees should we plant?)
3. How to prepare the soil for planting
4. Is there a monthly/ quarterly/ annual care routine we need to follow?
Thank you so much for any tips...

Julie