First you need to decide what fruit you would like..apples, pears, plums etc.
And then the fun starts...deciding the varieties. ;D
It helps if you have some idea what kind of space you have available to fill.
As for buying trees..well that is up to you..'normal' shops trees are perfectly fine, but the varieties they have on offer are very limited and tend to be commercial varieties..same that are sold as fruit in supermarkets.
If you buy from nurseries..yes, they are bit more expensive, but often you have hundreds of different sort to choose from and more saying what size the trees will grow into.
Almost all fruit trees are crafted into different rootstock..generally it is the used rootstock that dominate how large that tree grow.
M26 is semi-drawfing rootstock and the tree crafted on that will have potential to grow up to 4 metres in 10 years. How many can you fit in?
Let say 2..then you need to look varieties that tickles you fancy....take note that they have similar flowering time or look for polination croup number/letter.
That's about it... ::) I know it sounds bit complicated, but it does help to pay attention and do some home work.
Maybe you could spend hour or two just reading about it first..getting some idea what you would like so when you go out to hunt your bargain you are aware of some basics ;) l'll post some links to have look at...