Personally I'd go with a crate if you're getting a new car, I prefer pups to be in crates. Admittedly your pup will be bigger than my pups, but I'd still go for a crate. As I recall a Mondeo hatchback boot is big.
Once you've got your estate then definitely tailgate guard and 'proper' dog guard. I had Barjo, very good, but there are others out there and there are quite a few folk who will make custom fit.
A friend of mine got one of the Halfords Advanced Mesh Headrest Dog Guards, I'm quite impressed with it, although it is pricey. However she managed to get hers from someone on Agilitynet for £20. I think the ones that are tension fitted floor to ceiling are quite useless - they get knocked so move out of position.
Good sites to look at for 2nd hand (and for tailgate & guard when you get around to it):
http://www.agilitynet.co.uk/clutch/fleamarket.htmhttp://www.obedienceuk.com/shop/products.asp?id=2&cat=ForSalehttp://www.obedienceuk.com/shop/products.asp?id=2&cat=ForSalehttp://www.dfordog.co.uk/wanted_forsale.htmand of course check the Bring & Buy on this site and e-Bay.