I went from Z4MC to M6 convertible, was a great move for me.
They are ridiculously good VFM.
imperative options- DAB, Piano Black interior, FULL leather, not extended (VERY important), comfort access, TV.
Is there a particular car you have in mind? Don`t buy off anyone except BMW and try and push for a 2 year warranty.
Ref running costs; they aren`t cheap cars to run. I`m averaging 16mpg and thats with a fair bit of motorway cruising. Tyres: don`t run them on cheap rubber, PS2s work so well on the M6, Continentals struggle with traction. PS2s £350 each on rears, £260ish on fronts, reckon on changing every 14,000 miles or so.
Servicing is I drive led, oil service £250ish, Insp. 1 £800ish, insp 2 £1200ish. Mine needs spark plugs and brake fluid change in Jan, again all advised by idrive.
If you are coming from a Z4M reckon on adding half as much again into running the thing.
Truly do it though, fantastic car.