If you are thinking of growing fruit against a wall, cordons planted at 45 degrees to the horizontal take up very little room and each give a small crop of fruit quite quickly whereas a single espalier will take up a large amount of space and take several years of training to establish but will then give a large crop in the long term. If you are thinking of growing several varieties of apple, choose varieties that crop at different times.
I would certainly consider waiting until spring and getting much cheaper supermarket apple or pear trees (this year Tesco sold at £6 or 2 for £10). Although they offer a much less restricted range of varieties and rootstocks than a specialist supplier, carefully selected plants should still crop well. These are sold as feathered maidens, that is two-year old plants. If you choose this route, check the quality of the rootball (which is in peat and enclosed in a small plastic bag and cannot be inspected until you open it) and return the whole plant if it is unsatisfactory. In this way, you should get a good quality plant which will establish well.