i agree re: diesel as already stated; i think unless you're doing lots of miles its not worth the potential running costs and hassle
the swirl flap thing is easy to confirm; under the inlet manifold there is an actuator (like a wastegate actuator on a turbo) located at the back (under windscreen) and a rod from this which runs under the intake runners to the front. This is attached to the notorious flaps which basically divert air a different way around the inlet manifold at low revs (i think under 1300rpm on a 530d, not sure about others). The problem with them is the little screws whcih hold the flaps to the posts whcih they rotate around can fall out, so you end up with a metal or plastic flap trying to get into the engine.. the worst part is the screws; these are small enough to get past the valves without first being chopped/smashed up, and are always metal hence cause lots of damage!
removing them is not a major job, and doesn't really make much difference to the running of the car. I found the symptoms of removed flaps twice in 2yrs on my 530d, both times while driving slowly / idling after 4hrs+ at "motorway speeds" across france.. its like a roughness at idle which is a bit disconcerting at first, but is not a big deal. certainly less of a deal than the pistons getting smashed to pieces by screws
you can simply stick your hand underneath the rear of the inlet manifold to check if the actuator and rod is there, or if you're posh and have one of those mirrors on a telescopic stick, use that
simple, depreciation proof, timeless..
if you're paying someone to do the work, £250 to do all of the above is not too painful