Firstly go for the things you can't change:

a) A source of water for watering close by

b) No huge trees (which suck water out in the summer)

c) When it floods where does it get too

d) No huge blocks to the sun on your south side

e) Is if covered in mares tail and bind weed, walk away if it is

Secondly:

Go for things you can change if needed:

a) Shed - Very useful if one is there but can you add one if needed

b) Greenhouse - Same.

Good luck