Hi, well whether a diesl makes sens finally I guess is down to the milage you expect to cover.
I know new the 530d which I have was (I think from memory) about £5000 more than the 530i so it would mena a lot of miles to reap that back.
If you're intending to buy a 1 year old model, then the difference may not be as much but diesel are worth more secondhand anyway.
I've been told that about 90% of BMWs sold in the UK are diesel and in teh case of the 5 series, I'd have thoght he bulk of those will be 520ds.
Road tax is cheaper for a diesel compared to petrol as road tax is based upon CO2 emmissions.
For a used car, I'd have thought it best to look at any BMW dealer, they will have a selection of used cars whihc will still be under warranty. you may pay a bit more than a used example at a non BMW location but it's piece of mind knowing that it would have been checked out and you have the BMW network to back you up.
I've not really looked but I would think that msot under 1 year used 5 series are probbaly the dealer demo cars, I wouldn't think many wold be ex privtae onwers or off lease as thats a very expensive way to trading in as the car will ahve lost alot of money in that first year.
Try this
http://www.bmw.co.uk/bmwuk/bmwauc/detailed_search/