I would definitely recommend finding plants for the soil you have rather than trying to get soil for the plants you want. There are huge numbers of lovely plants for every soil, so whatever you've got you won't be very restricted.
If you're not there long (3 years in garden terms is quite short) and it is indeed large, please do pay attention to how the sun moves over the garden and where the dark shade, part shade, and full sun hits. That plus a little attention to the soil type you have (as the wise people mention above) in all your areas (as it can change quickly from one area to another), you will be up and going.....bulbs, plug plants and seeds are cheaper than full impact mature plants from nurseries if the budget is an issue.
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This is useful for seeing what you might have:
http://www.landis.org.uk/soilscapes/