My father is looking into a 2015/2016 era BMW 5 Series (F10?), he's not sure if the 520 or 530 is his preferred option but he does crunch a few miles per year (18-25k) so I'd assume that it would be a dag dag - happy to explain to him that 520i/530i is a better option if it is? He mainly drives to the golf course on the weekend so it needs to "look good in the carpark", I think it looks great anyway.
The 520i is very rare (and not very good either, not quick but not economical) and the 530i was discontinued at the facelift, so all are 13 plate or older now. The most common petrol option is the 528i whcih is a 4 cylinder turbo with 245bhp. They are very rare so its unlikely you'll come across one. If you particularly dislike diesel, it's a reasonable choice, but when I test drove one years back in an attempt to get away from driving a diesel I ended up with another 530d instead, so read into that what you will.
In 2014, the 2 litre diesel changed from the N Series to the B Series. The B Series is newer, slightly more powerful and hopefully more reliable. I think at this point I'd not want a 520d without the B Series engine.
Is this era of 5 series a good car for motorway cruising and general comfort and drive?
Absolutely! It's great for that.
I've seen some with a very small satnav and some with a much larger one in the dash, which spec comes with the larger? Is it the MSport?
The only trim level which comes with the Professional Navigation screen as standard is the Luxury - otherwise, its optional. So you'll need to pick a car with it fitted as an option if you want an M Sport. Even if you're not particularly going to use the nav its probably worth finding, it doesn't add a huge amount to the price and the silly little screen spoils the interior a bit.
If anyone could suggest some "must have" options in the spec list so he can refine his choice that would be great, I know there are a few 5 series fans on the forum. I have almost zero knowledge of BMWs aside from the no indicators trope so seeking help from you wise ones.
With the facelift, you don't need THAT many options. Obviously a fully loaded car is a wonderful thing but these are few and far between and almost everything you'd actually want is standard anyway (heated seats, better instrument cluster on post April 2014 cars, dual zone climate etc). So I'd probably stick Professional Navigation as the only must have - everything else is nice to have if you can find it but don't compromise on condition or engine choice to get more spec beyond Pro Nav. It's unlikely you'll find one with only Pro nav anyway, if somebody optioned Pro nav they likely added a few more bits too.