Dont understand why you are getting rid!
I just upgraded to a 5 door GTI ED30, since my previous car was a small 3 door. I have two young kids (aged 3 and 1) and one of my main reasons for choosing the Golf was that I didnt want anything too big, but something with enough room to put the kids in, a buggy in the boot, and still have space for any shopping/bags etc. The Golf was perfect for me!
I have a baby seat and a booster seat in the back (helps when parking in the open spaces at Tesco

) and can fit a double buggy in the boot, and still have some spare space for shopping.
Obviously your needs will differ to mine, and the 530d is a great looking car IMO, but was far too big for me! Dont think you will be making any cost savings with it though, even if it is a Diesel - may even end up costing more not just to run, but in parts/servicing etc?