Depends how much you can get off the price on outright purchase, if you don't think you will lose £12k by buying now and selling in 3 years time, that might be a better option (which if you really can buy for £19.5k, is definitely the way to go)
trust me you will loose 12k mine is 2 years old and has lost 12k, i did mine on a pcp which with the current climate was the best decision