5 series advice

Hi Robert and welcome to the forum.

Why a diesel? 

Will your wife be getting on for doing 20,000 miles a year?

The benifits of a frugal diesel will only come into play when you start doing over 15,000 miles per year.

You are looking at buying an E60.  By this time BMW had sorted the turbo failure problem so you should have little to worry about in terms of diesel reliability.  My E60 has only let me down once in 25,000 miles and that was a small engine management problem that BMW Fairbairns sorted quickly.  The runflat tyres are pricey so make sure that the ones you look at have plenty of tread on them.  They still suffer the odd electrical gremlin but no where near as bad as the E39 (previous 5 er) had.  Make sure the iDrive has plenty of miles to go before anything needs attention.  The DSC/ABS units can play up and can cost four figures to fix, I had mine replaced under warranty on my first E60.  I am led to believe that there has been some issues with parts of the rear suspension (rubber bushes I think) which can wear out but I haven't found this on mine.

100,000 miles is very high for a 5 year old car.  You want to make sure that you get one that has the most amount of toys/highest possible spec that you can get.  Ideally you want leather seats as a minimum.  I would take it in auto which if you are going for, get the autobox serviced after purchase.

The E60 has isofix kiddie seat points in the rear which will be a benefit if your kids are still in kiddie seats.

I personally wouldn't have a diesel as they are smelly, noisey and dynamically un responsive to drive.  Atleast you aren't being tempted by a 520d!  The only diesel E60 to buy would be the twin turbo 535d but they are still pricey.  Do a search on Youtube for "Top Gear BMW 535d" for a review of it.  My work don't allow access to Youtube so I can't give you a link.

Andrew

At motorway cruising speeds I get 34.4 mpg at 70mph, which for me is more economical than my old 523i was on the M'ways and more econimcal than my old Vauxhall Senator was.  I suppose I'm used to sub 30 mpg average fuel consumption.  If I cruise at ahem, cough, nearer 80 I get 32 mpg.   Diesels will get into the 40s I would have thought but will be noiser.

Is that camping to France pulling a trailer or just a heavily loaded car?

One trip a year shouldn't be justification for the type of car.  You need to be able to live with it three six five including trips to Tesco and the like.